From: Wanpeng Li <[email protected]>
'Commit d0659d946be0 ("KVM: x86: add option to advance tscdeadline
hrtimer expiration")' advances the tscdeadline (the timer is emulated
by hrtimer) expiration in order that the latency which is incurred
by hypervisor (apic_timer_fn -> vmentry) can be avoided. This patch
adds the advance tscdeadline expiration support to which the tscdeadline
timer is emulated by VMX preemption timer to reduce the hypervisor
lantency (handle_preemption_timer -> vmentry). clockevents infrastruture
can program minimum delay if hrtimer feeds a expiration in the past,
we set delta_tsc to 1(which will be converted to 0 before vmentry)
which can lead to an immediately vmexit when delta_tsc is not bigger
than advance ns.
This patch can reduce ~63% latency (~4450 cycles to ~1660 cycles on
a haswell desktop) for kvm-unit-tests/tscdeadline_latency when testing
busy waits.
Cc: Paolo Bonzini <[email protected]>
Cc: Radim Krčmář <[email protected]>
Signed-off-by: Wanpeng Li <[email protected]>
---
arch/x86/kvm/vmx.c | 8 +++++++-
arch/x86/kvm/x86.c | 1 +
2 files changed, 8 insertions(+), 1 deletion(-)
diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index e50beb7..d8b5110 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -12435,7 +12435,7 @@ static inline int u64_shl_div_u64(u64 a, unsigned int shift,
static int vmx_set_hv_timer(struct kvm_vcpu *vcpu, u64 guest_deadline_tsc)
{
struct vcpu_vmx *vmx;
- u64 tscl, guest_tscl, delta_tsc;
+ u64 tscl, guest_tscl, delta_tsc, lapic_timer_advance_cycles;
if (kvm_mwait_in_guest(vcpu->kvm))
return -EOPNOTSUPP;
@@ -12444,6 +12444,12 @@ static int vmx_set_hv_timer(struct kvm_vcpu *vcpu, u64 guest_deadline_tsc)
tscl = rdtsc();
guest_tscl = kvm_read_l1_tsc(vcpu, tscl);
delta_tsc = max(guest_deadline_tsc, guest_tscl) - guest_tscl;
+ lapic_timer_advance_cycles = nsec_to_cycles(vcpu, lapic_timer_advance_ns);
+
+ if (delta_tsc > lapic_timer_advance_cycles)
+ delta_tsc -= lapic_timer_advance_cycles;
+ else
+ delta_tsc = 1;
/* Convert to host delta tsc if tsc scaling is enabled */
if (vcpu->arch.tsc_scaling_ratio != kvm_default_tsc_scaling_ratio &&
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 22bd20f..d8abd96 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -138,6 +138,7 @@ module_param(tsc_tolerance_ppm, uint, S_IRUGO | S_IWUSR);
/* lapic timer advance (tscdeadline mode only) in nanoseconds */
unsigned int __read_mostly lapic_timer_advance_ns = 0;
module_param(lapic_timer_advance_ns, uint, S_IRUGO | S_IWUSR);
+EXPORT_SYMBOL_GPL(lapic_timer_advance_ns);
static bool __read_mostly vector_hashing = true;
module_param(vector_hashing, bool, S_IRUGO);
--
2.7.4
2018-05-29 14:53+0800, Wanpeng Li:
> From: Wanpeng Li <[email protected]>
>
> 'Commit d0659d946be0 ("KVM: x86: add option to advance tscdeadline
> hrtimer expiration")' advances the tscdeadline (the timer is emulated
> by hrtimer) expiration in order that the latency which is incurred
> by hypervisor (apic_timer_fn -> vmentry) can be avoided. This patch
> adds the advance tscdeadline expiration support to which the tscdeadline
> timer is emulated by VMX preemption timer to reduce the hypervisor
> lantency (handle_preemption_timer -> vmentry). clockevents infrastruture
> can program minimum delay if hrtimer feeds a expiration in the past,
> we set delta_tsc to 1(which will be converted to 0 before vmentry)
> which can lead to an immediately vmexit when delta_tsc is not bigger
> than advance ns.
>
> This patch can reduce ~63% latency (~4450 cycles to ~1660 cycles on
> a haswell desktop) for kvm-unit-tests/tscdeadline_latency when testing
> busy waits.
>
> Cc: Paolo Bonzini <[email protected]>
> Cc: Radim Krčmář <[email protected]>
> Signed-off-by: Wanpeng Li <[email protected]>
> ---
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> @@ -12444,6 +12444,12 @@ static int vmx_set_hv_timer(struct kvm_vcpu *vcpu, u64 guest_deadline_tsc)
> tscl = rdtsc();
> guest_tscl = kvm_read_l1_tsc(vcpu, tscl);
> delta_tsc = max(guest_deadline_tsc, guest_tscl) - guest_tscl;
> + lapic_timer_advance_cycles = nsec_to_cycles(vcpu, lapic_timer_advance_ns);
> + if (delta_tsc > lapic_timer_advance_cycles)
> + delta_tsc -= lapic_timer_advance_cycles;
> + else
> + delta_tsc = 1;
Why don't we just "return 1" to say that the timer has expired?
I think "delta_tsc = 1" would just force an immediate VM exit and
a re-entry, which seems wasteful as we could just be delaying the entry
until the deadline has really passed,
thanks.
2018-05-29 16:23+0200, Radim Krčmář:
> 2018-05-29 14:53+0800, Wanpeng Li:
> > From: Wanpeng Li <[email protected]>
> >
> > 'Commit d0659d946be0 ("KVM: x86: add option to advance tscdeadline
> > hrtimer expiration")' advances the tscdeadline (the timer is emulated
> > by hrtimer) expiration in order that the latency which is incurred
> > by hypervisor (apic_timer_fn -> vmentry) can be avoided. This patch
> > adds the advance tscdeadline expiration support to which the tscdeadline
> > timer is emulated by VMX preemption timer to reduce the hypervisor
> > lantency (handle_preemption_timer -> vmentry). clockevents infrastruture
> > can program minimum delay if hrtimer feeds a expiration in the past,
> > we set delta_tsc to 1(which will be converted to 0 before vmentry)
> > which can lead to an immediately vmexit when delta_tsc is not bigger
> > than advance ns.
> >
> > This patch can reduce ~63% latency (~4450 cycles to ~1660 cycles on
> > a haswell desktop) for kvm-unit-tests/tscdeadline_latency when testing
> > busy waits.
> >
> > Cc: Paolo Bonzini <[email protected]>
> > Cc: Radim Krčmář <[email protected]>
> > Signed-off-by: Wanpeng Li <[email protected]>
> > ---
> > diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> > @@ -12444,6 +12444,12 @@ static int vmx_set_hv_timer(struct kvm_vcpu *vcpu, u64 guest_deadline_tsc)
> > tscl = rdtsc();
> > guest_tscl = kvm_read_l1_tsc(vcpu, tscl);
> > delta_tsc = max(guest_deadline_tsc, guest_tscl) - guest_tscl;
> > + lapic_timer_advance_cycles = nsec_to_cycles(vcpu, lapic_timer_advance_ns);
> > + if (delta_tsc > lapic_timer_advance_cycles)
> > + delta_tsc -= lapic_timer_advance_cycles;
> > + else
> > + delta_tsc = 1;
>
> Why don't we just "return 1" to say that the timer has expired?
This case might be rare, so setting delta_tsc = 0 would be safer.
On 29/05/2018 16:31, Radim Krčmář wrote:
> 2018-05-29 16:23+0200, Radim Krčmář:
>> 2018-05-29 14:53+0800, Wanpeng Li:
>>> From: Wanpeng Li <[email protected]>
>>>
>>> 'Commit d0659d946be0 ("KVM: x86: add option to advance tscdeadline
>>> hrtimer expiration")' advances the tscdeadline (the timer is emulated
>>> by hrtimer) expiration in order that the latency which is incurred
>>> by hypervisor (apic_timer_fn -> vmentry) can be avoided. This patch
>>> adds the advance tscdeadline expiration support to which the tscdeadline
>>> timer is emulated by VMX preemption timer to reduce the hypervisor
>>> lantency (handle_preemption_timer -> vmentry). clockevents infrastruture
>>> can program minimum delay if hrtimer feeds a expiration in the past,
>>> we set delta_tsc to 1(which will be converted to 0 before vmentry)
>>> which can lead to an immediately vmexit when delta_tsc is not bigger
>>> than advance ns.
>>>
>>> This patch can reduce ~63% latency (~4450 cycles to ~1660 cycles on
>>> a haswell desktop) for kvm-unit-tests/tscdeadline_latency when testing
>>> busy waits.
>>>
>>> Cc: Paolo Bonzini <[email protected]>
>>> Cc: Radim Krčmář <[email protected]>
>>> Signed-off-by: Wanpeng Li <[email protected]>
>>> ---
>>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>>> @@ -12444,6 +12444,12 @@ static int vmx_set_hv_timer(struct kvm_vcpu *vcpu, u64 guest_deadline_tsc)
>>> tscl = rdtsc();
>>> guest_tscl = kvm_read_l1_tsc(vcpu, tscl);
>>> delta_tsc = max(guest_deadline_tsc, guest_tscl) - guest_tscl;
>>> + lapic_timer_advance_cycles = nsec_to_cycles(vcpu, lapic_timer_advance_ns);
>>> + if (delta_tsc > lapic_timer_advance_cycles)
>>> + delta_tsc -= lapic_timer_advance_cycles;
>>> + else
>>> + delta_tsc = 1;
>>
>> Why don't we just "return 1" to say that the timer has expired?
>
> This case might be rare, so setting delta_tsc = 0 would be safer.
Queued with this change. Indeed this case matches vmx_arm_hv_timer so
it's preferrable.
Paolo
On Wed, 30 May 2018 at 01:08, Paolo Bonzini <[email protected]> wrote:
> On 29/05/2018 16:31, Radim Krčmář wrote:
> > 2018-05-29 16:23+0200, Radim Krčmář:
> >> 2018-05-29 14:53+0800, Wanpeng Li:
> >>> From: Wanpeng Li <[email protected]>
> >>>
> >>> 'Commit d0659d946be0 ("KVM: x86: add option to advance tscdeadline
> >>> hrtimer expiration")' advances the tscdeadline (the timer is emulated
> >>> by hrtimer) expiration in order that the latency which is incurred
> >>> by hypervisor (apic_timer_fn -> vmentry) can be avoided. This patch
> >>> adds the advance tscdeadline expiration support to which the
tscdeadline
> >>> timer is emulated by VMX preemption timer to reduce the hypervisor
> >>> lantency (handle_preemption_timer -> vmentry). clockevents
infrastruture
> >>> can program minimum delay if hrtimer feeds a expiration in the past,
> >>> we set delta_tsc to 1(which will be converted to 0 before vmentry)
> >>> which can lead to an immediately vmexit when delta_tsc is not bigger
> >>> than advance ns.
> >>>
> >>> This patch can reduce ~63% latency (~4450 cycles to ~1660 cycles on
> >>> a haswell desktop) for kvm-unit-tests/tscdeadline_latency when testing
> >>> busy waits.
> >>>
> >>> Cc: Paolo Bonzini <[email protected]>
> >>> Cc: Radim Krčmář <[email protected]>
> >>> Signed-off-by: Wanpeng Li <[email protected]>
> >>> ---
> >>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> >>> @@ -12444,6 +12444,12 @@ static int vmx_set_hv_timer(struct kvm_vcpu
*vcpu, u64 guest_deadline_tsc)
> >>> tscl = rdtsc();
> >>> guest_tscl = kvm_read_l1_tsc(vcpu, tscl);
> >>> delta_tsc = max(guest_deadline_tsc, guest_tscl) - guest_tscl;
> >>> + lapic_timer_advance_cycles = nsec_to_cycles(vcpu,
lapic_timer_advance_ns);
> >>> + if (delta_tsc > lapic_timer_advance_cycles)
> >>> + delta_tsc -= lapic_timer_advance_cycles;
> >>> + else
> >>> + delta_tsc = 1;
> >>
> >> Why don't we just "return 1" to say that the timer has expired?
> >
> > This case might be rare, so setting delta_tsc = 0 would be safer.
> Queued with this change. Indeed this case matches vmx_arm_hv_timer so
> it's preferrable.
Agreed, thanks.
Regards,
Wanpeng Li
On Wed, 30 May 2018 at 01:08, Paolo Bonzini <[email protected]> wrote:
>
> On 29/05/2018 16:31, Radim Krčmář wrote:
> > 2018-05-29 16:23+0200, Radim Krčmář:
> >> 2018-05-29 14:53+0800, Wanpeng Li:
> >>> From: Wanpeng Li <[email protected]>
> >>>
> >>> 'Commit d0659d946be0 ("KVM: x86: add option to advance tscdeadline
> >>> hrtimer expiration")' advances the tscdeadline (the timer is emulated
> >>> by hrtimer) expiration in order that the latency which is incurred
> >>> by hypervisor (apic_timer_fn -> vmentry) can be avoided. This patch
> >>> adds the advance tscdeadline expiration support to which the tscdeadline
> >>> timer is emulated by VMX preemption timer to reduce the hypervisor
> >>> lantency (handle_preemption_timer -> vmentry). clockevents infrastruture
> >>> can program minimum delay if hrtimer feeds a expiration in the past,
> >>> we set delta_tsc to 1(which will be converted to 0 before vmentry)
> >>> which can lead to an immediately vmexit when delta_tsc is not bigger
> >>> than advance ns.
> >>>
> >>> This patch can reduce ~63% latency (~4450 cycles to ~1660 cycles on
> >>> a haswell desktop) for kvm-unit-tests/tscdeadline_latency when testing
> >>> busy waits.
> >>>
> >>> Cc: Paolo Bonzini <[email protected]>
> >>> Cc: Radim Krčmář <[email protected]>
> >>> Signed-off-by: Wanpeng Li <[email protected]>
> >>> ---
> >>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> >>> @@ -12444,6 +12444,12 @@ static int vmx_set_hv_timer(struct kvm_vcpu *vcpu, u64 guest_deadline_tsc)
> >>> tscl = rdtsc();
> >>> guest_tscl = kvm_read_l1_tsc(vcpu, tscl);
> >>> delta_tsc = max(guest_deadline_tsc, guest_tscl) - guest_tscl;
> >>> + lapic_timer_advance_cycles = nsec_to_cycles(vcpu, lapic_timer_advance_ns);
> >>> + if (delta_tsc > lapic_timer_advance_cycles)
> >>> + delta_tsc -= lapic_timer_advance_cycles;
> >>> + else
> >>> + delta_tsc = 1;
> >>
> >> Why don't we just "return 1" to say that the timer has expired?
> >
> > This case might be rare, so setting delta_tsc = 0 would be safer.
>
> Queued with this change. Indeed this case matches vmx_arm_hv_timer so
The patch description should also be updated in kvm/queue I think.
Regards,
Wanpeng Li