From: Wanpeng Li <[email protected]>
IPI and Timer cause the main MSRs write vmexits in cloud environment
observation, let's optimize virtual IPI latency more aggressively to
inject target IPI as soon as possible.
Running kvm-unit-tests/vmexit.flat IPI testing on SKX server, disable
adaptive advance lapic timer and adaptive halt-polling to avoid the
interference, this patch can give another 7% improvement.
w/o fastpath -> fastpath 4238 -> 3543 16.4%
fastpath -> ultra fastpath 3543 -> 3293 7%
w/o fastpath -> ultra fastpath 4238 -> 3293 22.3%
This also revises the performance data in commit 1e9e2622a1 (KVM: VMX:
FIXED+PHYSICAL mode single target IPI fastpath), that testing adds
--overcommit cpu-pm=on to kvm-unit-tests guest which is unnecessary.
Tested-by: Haiwei Li <[email protected]>
Cc: Haiwei Li <[email protected]>
Signed-off-by: Wanpeng Li <[email protected]>
---
v1 -> v2:
* rebase on latest kvm/queue
* update patch description
arch/x86/include/asm/kvm_host.h | 6 +++---
arch/x86/kvm/svm/svm.c | 21 ++++++++++++++-------
arch/x86/kvm/vmx/vmx.c | 19 +++++++++++++------
arch/x86/kvm/x86.c | 4 ++--
4 files changed, 32 insertions(+), 18 deletions(-)
diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index c7da23a..e667cf3 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -1124,7 +1124,8 @@ struct kvm_x86_ops {
*/
void (*tlb_flush_guest)(struct kvm_vcpu *vcpu);
- void (*run)(struct kvm_vcpu *vcpu);
+ void (*run)(struct kvm_vcpu *vcpu,
+ enum exit_fastpath_completion *exit_fastpath);
int (*handle_exit)(struct kvm_vcpu *vcpu,
enum exit_fastpath_completion exit_fastpath);
int (*skip_emulated_instruction)(struct kvm_vcpu *vcpu);
@@ -1174,8 +1175,7 @@ struct kvm_x86_ops {
struct x86_instruction_info *info,
enum x86_intercept_stage stage,
struct x86_exception *exception);
- void (*handle_exit_irqoff)(struct kvm_vcpu *vcpu,
- enum exit_fastpath_completion *exit_fastpath);
+ void (*handle_exit_irqoff)(struct kvm_vcpu *vcpu);
int (*check_nested_events)(struct kvm_vcpu *vcpu);
void (*request_immediate_exit)(struct kvm_vcpu *vcpu);
diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 27f4684..c019332 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -3283,9 +3283,20 @@ static void svm_cancel_injection(struct kvm_vcpu *vcpu)
svm_complete_interrupts(svm);
}
+static enum exit_fastpath_completion svm_exit_handlers_fastpath(struct kvm_vcpu *vcpu)
+{
+ if (!is_guest_mode(vcpu) &&
+ to_svm(vcpu)->vmcb->control.exit_code == SVM_EXIT_MSR &&
+ to_svm(vcpu)->vmcb->control.exit_info_1)
+ return handle_fastpath_set_msr_irqoff(vcpu);
+
+ return EXIT_FASTPATH_NONE;
+}
+
bool __svm_vcpu_run(unsigned long vmcb_pa, unsigned long *regs);
-static void svm_vcpu_run(struct kvm_vcpu *vcpu)
+static void svm_vcpu_run(struct kvm_vcpu *vcpu,
+ enum exit_fastpath_completion *exit_fastpath)
{
struct vcpu_svm *svm = to_svm(vcpu);
@@ -3388,6 +3399,7 @@ static void svm_vcpu_run(struct kvm_vcpu *vcpu)
kvm_load_host_xsave_state(vcpu);
stgi();
+ *exit_fastpath = svm_exit_handlers_fastpath(vcpu);
/* Any pending NMI will happen here */
if (unlikely(svm->vmcb->control.exit_code == SVM_EXIT_NMI))
@@ -3719,13 +3731,8 @@ static int svm_check_intercept(struct kvm_vcpu *vcpu,
return ret;
}
-static void svm_handle_exit_irqoff(struct kvm_vcpu *vcpu,
- enum exit_fastpath_completion *exit_fastpath)
+static void svm_handle_exit_irqoff(struct kvm_vcpu *vcpu)
{
- if (!is_guest_mode(vcpu) &&
- to_svm(vcpu)->vmcb->control.exit_code == SVM_EXIT_MSR &&
- to_svm(vcpu)->vmcb->control.exit_info_1)
- *exit_fastpath = handle_fastpath_set_msr_irqoff(vcpu);
}
static void svm_sched_in(struct kvm_vcpu *vcpu, int cpu)
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 1d2bb57..61a1725 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -6354,8 +6354,7 @@ static void handle_external_interrupt_irqoff(struct kvm_vcpu *vcpu)
}
STACK_FRAME_NON_STANDARD(handle_external_interrupt_irqoff);
-static void vmx_handle_exit_irqoff(struct kvm_vcpu *vcpu,
- enum exit_fastpath_completion *exit_fastpath)
+static void vmx_handle_exit_irqoff(struct kvm_vcpu *vcpu)
{
struct vcpu_vmx *vmx = to_vmx(vcpu);
@@ -6363,9 +6362,6 @@ static void vmx_handle_exit_irqoff(struct kvm_vcpu *vcpu,
handle_external_interrupt_irqoff(vcpu);
else if (vmx->exit_reason == EXIT_REASON_EXCEPTION_NMI)
handle_exception_nmi_irqoff(vmx);
- else if (!is_guest_mode(vcpu) &&
- vmx->exit_reason == EXIT_REASON_MSR_WRITE)
- *exit_fastpath = handle_fastpath_set_msr_irqoff(vcpu);
}
static bool vmx_has_emulated_msr(int index)
@@ -6570,9 +6566,19 @@ void vmx_update_host_rsp(struct vcpu_vmx *vmx, unsigned long host_rsp)
}
}
+static enum exit_fastpath_completion vmx_exit_handlers_fastpath(struct kvm_vcpu *vcpu)
+{
+ if (!is_guest_mode(vcpu) &&
+ to_vmx(vcpu)->exit_reason == EXIT_REASON_MSR_WRITE)
+ return handle_fastpath_set_msr_irqoff(vcpu);
+
+ return EXIT_FASTPATH_NONE;
+}
+
bool __vmx_vcpu_run(struct vcpu_vmx *vmx, unsigned long *regs, bool launched);
-static void vmx_vcpu_run(struct kvm_vcpu *vcpu)
+static void vmx_vcpu_run(struct kvm_vcpu *vcpu,
+ enum exit_fastpath_completion *exit_fastpath)
{
struct vcpu_vmx *vmx = to_vmx(vcpu);
unsigned long cr3, cr4;
@@ -6737,6 +6743,7 @@ static void vmx_vcpu_run(struct kvm_vcpu *vcpu)
vmx->idt_vectoring_info = 0;
vmx->exit_reason = vmx->fail ? 0xdead : vmcs_read32(VM_EXIT_REASON);
+ *exit_fastpath = vmx_exit_handlers_fastpath(vcpu);
if ((u16)vmx->exit_reason == EXIT_REASON_MCE_DURING_VMENTRY)
kvm_machine_check();
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 3089aa4..eed31e2 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -8409,7 +8409,7 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
vcpu->arch.switch_db_regs &= ~KVM_DEBUGREG_RELOAD;
}
- kvm_x86_ops.run(vcpu);
+ kvm_x86_ops.run(vcpu, &exit_fastpath);
/*
* Do this here before restoring debug registers on the host. And
@@ -8441,7 +8441,7 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
vcpu->mode = OUTSIDE_GUEST_MODE;
smp_wmb();
- kvm_x86_ops.handle_exit_irqoff(vcpu, &exit_fastpath);
+ kvm_x86_ops.handle_exit_irqoff(vcpu);
/*
* Consume any pending interrupts, including the possible source of
--
2.7.4
On 10/04/20 03:03, Wanpeng Li wrote:
> From: Wanpeng Li <[email protected]>
>
> IPI and Timer cause the main MSRs write vmexits in cloud environment
> observation, let's optimize virtual IPI latency more aggressively to
> inject target IPI as soon as possible.
>
> Running kvm-unit-tests/vmexit.flat IPI testing on SKX server, disable
> adaptive advance lapic timer and adaptive halt-polling to avoid the
> interference, this patch can give another 7% improvement.
>
> w/o fastpath -> fastpath 4238 -> 3543 16.4%
> fastpath -> ultra fastpath 3543 -> 3293 7%
> w/o fastpath -> ultra fastpath 4238 -> 3293 22.3%
>
> This also revises the performance data in commit 1e9e2622a1 (KVM: VMX:
> FIXED+PHYSICAL mode single target IPI fastpath), that testing adds
> --overcommit cpu-pm=on to kvm-unit-tests guest which is unnecessary.
>
> Tested-by: Haiwei Li <[email protected]>
> Cc: Haiwei Li <[email protected]>
> Signed-off-by: Wanpeng Li <[email protected]>
> ---
> v1 -> v2:
> * rebase on latest kvm/queue
> * update patch description
>
> arch/x86/include/asm/kvm_host.h | 6 +++---
> arch/x86/kvm/svm/svm.c | 21 ++++++++++++++-------
> arch/x86/kvm/vmx/vmx.c | 19 +++++++++++++------
> arch/x86/kvm/x86.c | 4 ++--
> 4 files changed, 32 insertions(+), 18 deletions(-)
That's less ugly than I expected. :D I'll queue it in the next week or
so. But even though the commit subject is cool, I'll change it to "KVM:
x86: move IPI fastpath inside kvm_x86_ops.run".
Thanks,
Paolo
On 10/04/20 17:35, Sean Christopherson wrote:
> IMO, this should come at the very end of vmx_vcpu_run(). At a minimum, it
> needs to be moved below the #MC handling and below
>
> if (vmx->fail || (vmx->exit_reason & VMX_EXIT_REASONS_FAILED_VMENTRY))
> return;
Why? It cannot run in any of those cases, since the vmx->exit_reason
won't match.
> KVM more or less assumes vmx->idt_vectoring_info is always valid, and it's
> not obvious that a generic fastpath call can safely run before
> vmx_complete_interrupts(), e.g. the kvm_clear_interrupt_queue() call.
Not KVM, rather vmx.c. You're right about a generic fastpath, but in
this case kvm_irq_delivery_to_apic_fast is not touching VMX state; even
if you have a self-IPI, the modification of vCPU state is only scheduled
here and will happen later via either kvm_x86_ops.sync_pir_to_irr or
KVM_REQ_EVENT.
Paolo
On Fri, Apr 10, 2020 at 09:03:27AM +0800, Wanpeng Li wrote:
> From: Wanpeng Li <[email protected]>
>
> IPI and Timer cause the main MSRs write vmexits in cloud environment
> observation, let's optimize virtual IPI latency more aggressively to
> inject target IPI as soon as possible.
>
> Running kvm-unit-tests/vmexit.flat IPI testing on SKX server, disable
> adaptive advance lapic timer and adaptive halt-polling to avoid the
> interference, this patch can give another 7% improvement.
>
> w/o fastpath -> fastpath 4238 -> 3543 16.4%
> fastpath -> ultra fastpath 3543 -> 3293 7%
> w/o fastpath -> ultra fastpath 4238 -> 3293 22.3%
>
> This also revises the performance data in commit 1e9e2622a1 (KVM: VMX:
> FIXED+PHYSICAL mode single target IPI fastpath), that testing adds
> --overcommit cpu-pm=on to kvm-unit-tests guest which is unnecessary.
>
> Tested-by: Haiwei Li <[email protected]>
> Cc: Haiwei Li <[email protected]>
> Signed-off-by: Wanpeng Li <[email protected]>
> ---
> v1 -> v2:
> * rebase on latest kvm/queue
> * update patch description
>
> arch/x86/include/asm/kvm_host.h | 6 +++---
> arch/x86/kvm/svm/svm.c | 21 ++++++++++++++-------
> arch/x86/kvm/vmx/vmx.c | 19 +++++++++++++------
> arch/x86/kvm/x86.c | 4 ++--
> 4 files changed, 32 insertions(+), 18 deletions(-)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index c7da23a..e667cf3 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1124,7 +1124,8 @@ struct kvm_x86_ops {
> */
> void (*tlb_flush_guest)(struct kvm_vcpu *vcpu);
>
> - void (*run)(struct kvm_vcpu *vcpu);
> + void (*run)(struct kvm_vcpu *vcpu,
> + enum exit_fastpath_completion *exit_fastpath);
> int (*handle_exit)(struct kvm_vcpu *vcpu,
> enum exit_fastpath_completion exit_fastpath);
> int (*skip_emulated_instruction)(struct kvm_vcpu *vcpu);
> @@ -1174,8 +1175,7 @@ struct kvm_x86_ops {
> struct x86_instruction_info *info,
> enum x86_intercept_stage stage,
> struct x86_exception *exception);
> - void (*handle_exit_irqoff)(struct kvm_vcpu *vcpu,
> - enum exit_fastpath_completion *exit_fastpath);
> + void (*handle_exit_irqoff)(struct kvm_vcpu *vcpu);
>
> int (*check_nested_events)(struct kvm_vcpu *vcpu);
> void (*request_immediate_exit)(struct kvm_vcpu *vcpu);
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 27f4684..c019332 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -3283,9 +3283,20 @@ static void svm_cancel_injection(struct kvm_vcpu *vcpu)
> svm_complete_interrupts(svm);
> }
>
> +static enum exit_fastpath_completion svm_exit_handlers_fastpath(struct kvm_vcpu *vcpu)
> +{
> + if (!is_guest_mode(vcpu) &&
> + to_svm(vcpu)->vmcb->control.exit_code == SVM_EXIT_MSR &&
> + to_svm(vcpu)->vmcb->control.exit_info_1)
> + return handle_fastpath_set_msr_irqoff(vcpu);
> +
> + return EXIT_FASTPATH_NONE;
> +}
> +
> bool __svm_vcpu_run(unsigned long vmcb_pa, unsigned long *regs);
>
> -static void svm_vcpu_run(struct kvm_vcpu *vcpu)
> +static void svm_vcpu_run(struct kvm_vcpu *vcpu,
> + enum exit_fastpath_completion *exit_fastpath)
> {
> struct vcpu_svm *svm = to_svm(vcpu);
>
> @@ -3388,6 +3399,7 @@ static void svm_vcpu_run(struct kvm_vcpu *vcpu)
> kvm_load_host_xsave_state(vcpu);
> stgi();
>
> + *exit_fastpath = svm_exit_handlers_fastpath(vcpu);
> /* Any pending NMI will happen here */
>
> if (unlikely(svm->vmcb->control.exit_code == SVM_EXIT_NMI))
> @@ -3719,13 +3731,8 @@ static int svm_check_intercept(struct kvm_vcpu *vcpu,
> return ret;
> }
>
> -static void svm_handle_exit_irqoff(struct kvm_vcpu *vcpu,
> - enum exit_fastpath_completion *exit_fastpath)
> +static void svm_handle_exit_irqoff(struct kvm_vcpu *vcpu)
> {
> - if (!is_guest_mode(vcpu) &&
> - to_svm(vcpu)->vmcb->control.exit_code == SVM_EXIT_MSR &&
> - to_svm(vcpu)->vmcb->control.exit_info_1)
> - *exit_fastpath = handle_fastpath_set_msr_irqoff(vcpu);
> }
>
> static void svm_sched_in(struct kvm_vcpu *vcpu, int cpu)
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 1d2bb57..61a1725 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -6354,8 +6354,7 @@ static void handle_external_interrupt_irqoff(struct kvm_vcpu *vcpu)
> }
> STACK_FRAME_NON_STANDARD(handle_external_interrupt_irqoff);
>
> -static void vmx_handle_exit_irqoff(struct kvm_vcpu *vcpu,
> - enum exit_fastpath_completion *exit_fastpath)
> +static void vmx_handle_exit_irqoff(struct kvm_vcpu *vcpu)
> {
> struct vcpu_vmx *vmx = to_vmx(vcpu);
>
> @@ -6363,9 +6362,6 @@ static void vmx_handle_exit_irqoff(struct kvm_vcpu *vcpu,
> handle_external_interrupt_irqoff(vcpu);
> else if (vmx->exit_reason == EXIT_REASON_EXCEPTION_NMI)
> handle_exception_nmi_irqoff(vmx);
> - else if (!is_guest_mode(vcpu) &&
> - vmx->exit_reason == EXIT_REASON_MSR_WRITE)
> - *exit_fastpath = handle_fastpath_set_msr_irqoff(vcpu);
> }
>
> static bool vmx_has_emulated_msr(int index)
> @@ -6570,9 +6566,19 @@ void vmx_update_host_rsp(struct vcpu_vmx *vmx, unsigned long host_rsp)
> }
> }
>
> +static enum exit_fastpath_completion vmx_exit_handlers_fastpath(struct kvm_vcpu *vcpu)
> +{
> + if (!is_guest_mode(vcpu) &&
> + to_vmx(vcpu)->exit_reason == EXIT_REASON_MSR_WRITE)
Bad indentation.
> + return handle_fastpath_set_msr_irqoff(vcpu);
> +
> + return EXIT_FASTPATH_NONE;
> +}
> +
> bool __vmx_vcpu_run(struct vcpu_vmx *vmx, unsigned long *regs, bool launched);
>
> -static void vmx_vcpu_run(struct kvm_vcpu *vcpu)
> +static void vmx_vcpu_run(struct kvm_vcpu *vcpu,
> + enum exit_fastpath_completion *exit_fastpath)
Why pass a pointer instead of returning the enum?
> {
> struct vcpu_vmx *vmx = to_vmx(vcpu);
> unsigned long cr3, cr4;
> @@ -6737,6 +6743,7 @@ static void vmx_vcpu_run(struct kvm_vcpu *vcpu)
> vmx->idt_vectoring_info = 0;
>
> vmx->exit_reason = vmx->fail ? 0xdead : vmcs_read32(VM_EXIT_REASON);
> + *exit_fastpath = vmx_exit_handlers_fastpath(vcpu);
IMO, this should come at the very end of vmx_vcpu_run(). At a minimum, it
needs to be moved below the #MC handling and below
if (vmx->fail || (vmx->exit_reason & VMX_EXIT_REASONS_FAILED_VMENTRY))
return;
KVM more or less assumes vmx->idt_vectoring_info is always valid, and it's
not obvious that a generic fastpath call can safely run before
vmx_complete_interrupts(), e.g. the kvm_clear_interrupt_queue() call.
In a normal scenario, the added latency is <50 cycles. ~30 for the VMREAD
of IDT_VECTORING_INFO_FIELD, a handful of zeroing instructions, and a few
CMP+Jcc style uops to skip NMI blocking and interrupt completion.
And if the result is returned, it means VMX won't need a local variable, e.g.:
vmx->exit_reason = vmx->fail ? 0xdead : vmcs_read32(VM_EXIT_REASON);
if ((u16)vmx->exit_reason == EXIT_REASON_MCE_DURING_VMENTRY)
kvm_machine_check();
if (vmx->fail || (vmx->exit_reason & VMX_EXIT_REASONS_FAILED_VMENTRY))
return EXIT_FASTPATH_NONE;
vmx->loaded_vmcs->launched = 1;
vmx->idt_vectoring_info = vmcs_read32(IDT_VECTORING_INFO_FIELD);
vmx_recover_nmi_blocking(vmx);
vmx_complete_interrupts(vmx);
return vmx_exit_handlers_fastpath(vcpu);
}
> if ((u16)vmx->exit_reason == EXIT_REASON_MCE_DURING_VMENTRY)
> kvm_machine_check();
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 3089aa4..eed31e2 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -8409,7 +8409,7 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
> vcpu->arch.switch_db_regs &= ~KVM_DEBUGREG_RELOAD;
> }
>
> - kvm_x86_ops.run(vcpu);
> + kvm_x86_ops.run(vcpu, &exit_fastpath);
Pretty sre
>
> /*
> * Do this here before restoring debug registers on the host. And
> @@ -8441,7 +8441,7 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
> vcpu->mode = OUTSIDE_GUEST_MODE;
> smp_wmb();
>
> - kvm_x86_ops.handle_exit_irqoff(vcpu, &exit_fastpath);
> + kvm_x86_ops.handle_exit_irqoff(vcpu);
>
> /*
> * Consume any pending interrupts, including the possible source of
> --
> 2.7.4
>
On Fri, Apr 10, 2020 at 05:50:35PM +0200, Paolo Bonzini wrote:
> On 10/04/20 17:35, Sean Christopherson wrote:
> > IMO, this should come at the very end of vmx_vcpu_run(). At a minimum, it
> > needs to be moved below the #MC handling and below
> >
> > if (vmx->fail || (vmx->exit_reason & VMX_EXIT_REASONS_FAILED_VMENTRY))
> > return;
>
> Why? It cannot run in any of those cases, since the vmx->exit_reason
> won't match.
#MC and consistency checks should have "priority" over everything else.
That there isn't actually a conflict is irrelevant IMO. And it's something
that will likely confuse newbies (to VMX and/or KVM) as it won't be obvious
that the motivation was to shave a few cycles, e.g. versus some corner case
where the fastpath handling does something meaningful even on failure.
> > KVM more or less assumes vmx->idt_vectoring_info is always valid, and it's
> > not obvious that a generic fastpath call can safely run before
> > vmx_complete_interrupts(), e.g. the kvm_clear_interrupt_queue() call.
>
> Not KVM, rather vmx.c. You're right about a generic fastpath, but in
> this case kvm_irq_delivery_to_apic_fast is not touching VMX state; even
> if you have a self-IPI, the modification of vCPU state is only scheduled
> here and will happen later via either kvm_x86_ops.sync_pir_to_irr or
> KVM_REQ_EVENT.
I think what I don't like is that the fast-IPI code is buried in a helper
that masquerades as a generic fastpath handler. If that's open-coded in
vmx_vcpu_run(), I'm ok with doing the fast-IPI handler immediately after
the failure checks.
And fast-IPI aside, the code could use a bit of optimization to prioritize
successful VM-Enter, which would slot in nicely as a prep patch. Patches
(should be) following.
IMO, this is more logically correct:
vmx->exit_reason = vmcs_read32(VM_EXIT_REASON);
if (unlikely((u16)vmx->exit_reason == EXIT_REASON_MCE_DURING_VMENTRY))
kvm_machine_check();
if (unlikely(vmx->exit_reason & VMX_EXIT_REASONS_FAILED_VMENTRY))
return EXIT_FASTPATH_NONE;
if (!is_guest_mode(vcpu) && vmx->exit_reason == EXIT_REASON_MSR_WRITE)
exit_fastpath = handle_fastpath_set_msr_irqoff(vcpu);
else
exit_fastpath = EXIT_FASTPATH_NONE;
And on my system, the compiler hoists fast-IPI above the #MC, e.g. moving
the fast-IPI down only adds a single macrofused uop, testb+jne for
FAILED_VMENTERY, to the code path.
0xffffffff81067d1d <+701>: vmread %rax,%rax
0xffffffff81067d20 <+704>: ja,pt 0xffffffff81067d2d <vmx_vcpu_run+717>
0xffffffff81067d23 <+707>: pushq $0x0
0xffffffff81067d25 <+709>: push %rax
0xffffffff81067d26 <+710>: callq 0xffffffff81071790 <vmread_error_trampoline>
0xffffffff81067d2b <+715>: pop %rax
0xffffffff81067d2c <+716>: pop %rax
0xffffffff81067d2d <+717>: test %eax,%eax
0xffffffff81067d2f <+719>: mov %eax,0x32b0(%rbp)
0xffffffff81067d35 <+725>: js 0xffffffff81067d5a <vmx_vcpu_run+762>
0xffffffff81067d37 <+727>: testb $0x20,0x2dc(%rbp)
0xffffffff81067d3e <+734>: jne 0xffffffff81067d49 <vmx_vcpu_run+745>
0xffffffff81067d40 <+736>: cmp $0x20,%eax
0xffffffff81067d43 <+739>: je 0xffffffff810686d4 <vmx_vcpu_run+3188> <-- fastpath handler
0xffffffff81067d49 <+745>: xor %ebx,%ebx
0xffffffff81067d4b <+747>: jmpq 0xffffffff81067e65 <vmx_vcpu_run+1029>
Mark the VM-Fail, VM-Exit on VM-Enter, and #MC on VM-Enter paths as
'unlikely' so as to improve code generation so that it favors successful
VM-Enter. The performance of successful VM-Enter is for more important,
irrespective of whether or not success is actually likely.
Signed-off-by: Sean Christopherson <[email protected]>
---
arch/x86/kvm/vmx/vmx.c | 11 ++++++++---
1 file changed, 8 insertions(+), 3 deletions(-)
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 1d2bb57f4ac4..a8402bed29e3 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -6736,11 +6736,16 @@ static void vmx_vcpu_run(struct kvm_vcpu *vcpu)
vmx->nested.nested_run_pending = 0;
vmx->idt_vectoring_info = 0;
- vmx->exit_reason = vmx->fail ? 0xdead : vmcs_read32(VM_EXIT_REASON);
- if ((u16)vmx->exit_reason == EXIT_REASON_MCE_DURING_VMENTRY)
+ if (unlikely(vmx->fail)) {
+ vmx->exit_reason = 0xdead;
+ return;
+ }
+
+ vmx->exit_reason = vmcs_read32(VM_EXIT_REASON);
+ if (unlikely((u16)vmx->exit_reason == EXIT_REASON_MCE_DURING_VMENTRY))
kvm_machine_check();
- if (vmx->fail || (vmx->exit_reason & VMX_EXIT_REASONS_FAILED_VMENTRY))
+ if (unlikely(vmx->exit_reason & VMX_EXIT_REASONS_FAILED_VMENTRY))
return;
vmx->loaded_vmcs->launched = 1;
--
2.26.0
On 10/04/20 19:47, Sean Christopherson wrote:
> If that's open-coded in vmx_vcpu_run(), I'm ok with doing the
> fast-IPI handler immediately after the failure checks.
>
> And fast-IPI aside, the code could use a bit of optimization to
> prioritize successful VM-Enter, which would slot in nicely as a prep
> patch.
Yes, I agree with these.
Paolo
On Sat, 11 Apr 2020 at 01:47, Sean Christopherson
<[email protected]> wrote:
>
> On Fri, Apr 10, 2020 at 05:50:35PM +0200, Paolo Bonzini wrote:
> > On 10/04/20 17:35, Sean Christopherson wrote:
> > > IMO, this should come at the very end of vmx_vcpu_run(). At a minimum, it
> > > needs to be moved below the #MC handling and below
> > >
> > > if (vmx->fail || (vmx->exit_reason & VMX_EXIT_REASONS_FAILED_VMENTRY))
> > > return;
> >
> > Why? It cannot run in any of those cases, since the vmx->exit_reason
> > won't match.
>
> #MC and consistency checks should have "priority" over everything else.
> That there isn't actually a conflict is irrelevant IMO. And it's something
> that will likely confuse newbies (to VMX and/or KVM) as it won't be obvious
> that the motivation was to shave a few cycles, e.g. versus some corner case
> where the fastpath handling does something meaningful even on failure.
>
> > > KVM more or less assumes vmx->idt_vectoring_info is always valid, and it's
> > > not obvious that a generic fastpath call can safely run before
> > > vmx_complete_interrupts(), e.g. the kvm_clear_interrupt_queue() call.
> >
> > Not KVM, rather vmx.c. You're right about a generic fastpath, but in
> > this case kvm_irq_delivery_to_apic_fast is not touching VMX state; even
> > if you have a self-IPI, the modification of vCPU state is only scheduled
> > here and will happen later via either kvm_x86_ops.sync_pir_to_irr or
> > KVM_REQ_EVENT.
>
> I think what I don't like is that the fast-IPI code is buried in a helper
> that masquerades as a generic fastpath handler. If that's open-coded in
> vmx_vcpu_run(), I'm ok with doing the fast-IPI handler immediately after
> the failure checks.
>
> And fast-IPI aside, the code could use a bit of optimization to prioritize
> successful VM-Enter, which would slot in nicely as a prep patch. Patches
> (should be) following.
Thanks for v3. :)
>
> IMO, this is more logically correct:
>
> vmx->exit_reason = vmcs_read32(VM_EXIT_REASON);
> if (unlikely((u16)vmx->exit_reason == EXIT_REASON_MCE_DURING_VMENTRY))
> kvm_machine_check();
>
> if (unlikely(vmx->exit_reason & VMX_EXIT_REASONS_FAILED_VMENTRY))
> return EXIT_FASTPATH_NONE;
>
> if (!is_guest_mode(vcpu) && vmx->exit_reason == EXIT_REASON_MSR_WRITE)
> exit_fastpath = handle_fastpath_set_msr_irqoff(vcpu);
> else
> exit_fastpath = EXIT_FASTPATH_NONE;
>
> And on my system, the compiler hoists fast-IPI above the #MC, e.g. moving
> the fast-IPI down only adds a single macrofused uop, testb+jne for
> FAILED_VMENTERY, to the code path.
>
> 0xffffffff81067d1d <+701>: vmread %rax,%rax
> 0xffffffff81067d20 <+704>: ja,pt 0xffffffff81067d2d <vmx_vcpu_run+717>
> 0xffffffff81067d23 <+707>: pushq $0x0
> 0xffffffff81067d25 <+709>: push %rax
> 0xffffffff81067d26 <+710>: callq 0xffffffff81071790 <vmread_error_trampoline>
> 0xffffffff81067d2b <+715>: pop %rax
> 0xffffffff81067d2c <+716>: pop %rax
> 0xffffffff81067d2d <+717>: test %eax,%eax
> 0xffffffff81067d2f <+719>: mov %eax,0x32b0(%rbp)
> 0xffffffff81067d35 <+725>: js 0xffffffff81067d5a <vmx_vcpu_run+762>
> 0xffffffff81067d37 <+727>: testb $0x20,0x2dc(%rbp)
> 0xffffffff81067d3e <+734>: jne 0xffffffff81067d49 <vmx_vcpu_run+745>
> 0xffffffff81067d40 <+736>: cmp $0x20,%eax
> 0xffffffff81067d43 <+739>: je 0xffffffff810686d4 <vmx_vcpu_run+3188> <-- fastpath handler
> 0xffffffff81067d49 <+745>: xor %ebx,%ebx
> 0xffffffff81067d4b <+747>: jmpq 0xffffffff81067e65 <vmx_vcpu_run+1029>
On Fri, 10 Apr 2020 at 23:17, Paolo Bonzini <[email protected]> wrote:
>
> On 10/04/20 03:03, Wanpeng Li wrote:
> > From: Wanpeng Li <[email protected]>
> >
> > IPI and Timer cause the main MSRs write vmexits in cloud environment
> > observation, let's optimize virtual IPI latency more aggressively to
> > inject target IPI as soon as possible.
> >
> > Running kvm-unit-tests/vmexit.flat IPI testing on SKX server, disable
> > adaptive advance lapic timer and adaptive halt-polling to avoid the
> > interference, this patch can give another 7% improvement.
> >
> > w/o fastpath -> fastpath 4238 -> 3543 16.4%
> > fastpath -> ultra fastpath 3543 -> 3293 7%
> > w/o fastpath -> ultra fastpath 4238 -> 3293 22.3%
> >
> > This also revises the performance data in commit 1e9e2622a1 (KVM: VMX:
> > FIXED+PHYSICAL mode single target IPI fastpath), that testing adds
> > --overcommit cpu-pm=on to kvm-unit-tests guest which is unnecessary.
> >
> > Tested-by: Haiwei Li <[email protected]>
> > Cc: Haiwei Li <[email protected]>
> > Signed-off-by: Wanpeng Li <[email protected]>
> > ---
> > v1 -> v2:
> > * rebase on latest kvm/queue
> > * update patch description
> >
> > arch/x86/include/asm/kvm_host.h | 6 +++---
> > arch/x86/kvm/svm/svm.c | 21 ++++++++++++++-------
> > arch/x86/kvm/vmx/vmx.c | 19 +++++++++++++------
> > arch/x86/kvm/x86.c | 4 ++--
> > 4 files changed, 32 insertions(+), 18 deletions(-)
>
> That's less ugly than I expected. :D I'll queue it in the next week or
> so. But even though the commit subject is cool, I'll change it to "KVM:
> x86: move IPI fastpath inside kvm_x86_ops.run".
Thanks.
Wanpeng