From: Wanpeng Li <[email protected]>
PMU is not exposed to guest by most of cloud providers since the bad performance
of PMU emulation and security concern. However, it calls perf_guest_switch_get_msrs()
and clear_atomic_switch_msr() unconditionally even if PMU is not exposed to the
guest before each vmentry.
~1.28% vmexit time reduced can be observed by kvm-unit-tests/vmexit.flat on my
SKX server.
Before patch:
vmcall 1559
After patch:
vmcall 1539
Signed-off-by: Wanpeng Li <[email protected]>
---
arch/x86/kvm/vmx/vmx.c | 3 +++
1 file changed, 3 insertions(+)
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 40b1e61..fd526c8 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -6441,6 +6441,9 @@ static void atomic_switch_perf_msrs(struct vcpu_vmx *vmx)
int i, nr_msrs;
struct perf_guest_switch_msr *msrs;
+ if (!vcpu_to_pmu(&vmx->vcpu)->version)
+ return;
+
msrs = perf_guest_get_msrs(&nr_msrs);
if (!msrs)
--
2.7.4
Wanpeng Li <[email protected]> writes:
> From: Wanpeng Li <[email protected]>
>
> PMU is not exposed to guest by most of cloud providers since the bad performance
> of PMU emulation and security concern. However, it calls perf_guest_switch_get_msrs()
> and clear_atomic_switch_msr() unconditionally even if PMU is not exposed to the
> guest before each vmentry.
>
> ~1.28% vmexit time reduced can be observed by kvm-unit-tests/vmexit.flat on my
> SKX server.
>
> Before patch:
> vmcall 1559
>
> After patch:
> vmcall 1539
>
> Signed-off-by: Wanpeng Li <[email protected]>
> ---
> arch/x86/kvm/vmx/vmx.c | 3 +++
> 1 file changed, 3 insertions(+)
>
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 40b1e61..fd526c8 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -6441,6 +6441,9 @@ static void atomic_switch_perf_msrs(struct vcpu_vmx *vmx)
> int i, nr_msrs;
> struct perf_guest_switch_msr *msrs;
>
> + if (!vcpu_to_pmu(&vmx->vcpu)->version)
> + return;
> +
> msrs = perf_guest_get_msrs(&nr_msrs);
>
> if (!msrs)
Personally, I'd prefer this to be expressed as
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 40b1e6138cd5..ace92076c90f 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -6567,7 +6567,9 @@ static void vmx_vcpu_run(struct kvm_vcpu *vcpu)
pt_guest_enter(vmx);
- atomic_switch_perf_msrs(vmx);
+ if (vcpu_to_pmu(&vmx->vcpu)->version)
+ atomic_switch_perf_msrs(vmx);
+
atomic_switch_umwait_control_msr(vmx);
if (enable_preemption_timer)
(which will likely produce the same code as atomic_switch_perf_msrs() is
likely inlined).
Also, (not knowing much about PMU), is
"vcpu_to_pmu(&vmx->vcpu)->version" check correct?
E.g. in intel_is_valid_msr() correct for Intel PMU or is it stated
somewhere that it is generic rule?
Also, speaking about cloud providers and the 'micro' nature of this
optimization, would it rather make sense to introduce a static branch
(the policy to disable vPMU is likely to be host wide, right)?
--
Vitaly
On Thu, 12 Mar 2020 at 18:36, Vitaly Kuznetsov <[email protected]> wrote:
>
> Wanpeng Li <[email protected]> writes:
>
> > From: Wanpeng Li <[email protected]>
> >
> > PMU is not exposed to guest by most of cloud providers since the bad performance
> > of PMU emulation and security concern. However, it calls perf_guest_switch_get_msrs()
> > and clear_atomic_switch_msr() unconditionally even if PMU is not exposed to the
> > guest before each vmentry.
> >
> > ~1.28% vmexit time reduced can be observed by kvm-unit-tests/vmexit.flat on my
> > SKX server.
> >
> > Before patch:
> > vmcall 1559
> >
> > After patch:
> > vmcall 1539
> >
> > Signed-off-by: Wanpeng Li <[email protected]>
> > ---
> > arch/x86/kvm/vmx/vmx.c | 3 +++
> > 1 file changed, 3 insertions(+)
> >
> > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> > index 40b1e61..fd526c8 100644
> > --- a/arch/x86/kvm/vmx/vmx.c
> > +++ b/arch/x86/kvm/vmx/vmx.c
> > @@ -6441,6 +6441,9 @@ static void atomic_switch_perf_msrs(struct vcpu_vmx *vmx)
> > int i, nr_msrs;
> > struct perf_guest_switch_msr *msrs;
> >
> > + if (!vcpu_to_pmu(&vmx->vcpu)->version)
> > + return;
> > +
> > msrs = perf_guest_get_msrs(&nr_msrs);
> >
> > if (!msrs)
>
> Personally, I'd prefer this to be expressed as
>
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 40b1e6138cd5..ace92076c90f 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -6567,7 +6567,9 @@ static void vmx_vcpu_run(struct kvm_vcpu *vcpu)
>
> pt_guest_enter(vmx);
>
> - atomic_switch_perf_msrs(vmx);
> + if (vcpu_to_pmu(&vmx->vcpu)->version)
> + atomic_switch_perf_msrs(vmx);
> +
I just hope the beautiful codes before, I testing this version before
sending out the patch, ~30 cycles can be saved which means that ~2%
vmexit time, will update in next version. Let's wait Paolo for other
opinions below.
Wanpeng
>
> Also, (not knowing much about PMU), is
> "vcpu_to_pmu(&vmx->vcpu)->version" check correct?
>
> E.g. in intel_is_valid_msr() correct for Intel PMU or is it stated
> somewhere that it is generic rule?
>
> Also, speaking about cloud providers and the 'micro' nature of this
> optimization, would it rather make sense to introduce a static branch
> (the policy to disable vPMU is likely to be host wide, right)?
>
> --
> Vitaly
>
On Thu, Mar 12, 2020 at 3:36 AM Vitaly Kuznetsov <[email protected]> wrote:
> Also, speaking about cloud providers and the 'micro' nature of this
> optimization, would it rather make sense to introduce a static branch
> (the policy to disable vPMU is likely to be host wide, right)?
Speaking for a cloud provider, no, the policy is not likely to be host-wide.
Hi Wanpeng,
On 2020/3/12 19:05, Wanpeng Li wrote:
> On Thu, 12 Mar 2020 at 18:36, Vitaly Kuznetsov <[email protected]> wrote:
>> Wanpeng Li <[email protected]> writes:
>>
>>> From: Wanpeng Li <[email protected]>
>>>
>>> PMU is not exposed to guest by most of cloud providers since the bad performance
>>> of PMU emulation and security concern. However, it calls perf_guest_switch_get_msrs()
>>> and clear_atomic_switch_msr() unconditionally even if PMU is not exposed to the
>>> guest before each vmentry.
>>>
>>> ~1.28% vmexit time reduced can be observed by kvm-unit-tests/vmexit.flat on my
>>> SKX server.
>>>
>>> Before patch:
>>> vmcall 1559
>>>
>>> After patch:
>>> vmcall 1539
>>>
>>> Signed-off-by: Wanpeng Li <[email protected]>
>>> ---
>>> arch/x86/kvm/vmx/vmx.c | 3 +++
>>> 1 file changed, 3 insertions(+)
>>>
>>> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
>>> index 40b1e61..fd526c8 100644
>>> --- a/arch/x86/kvm/vmx/vmx.c
>>> +++ b/arch/x86/kvm/vmx/vmx.c
>>> @@ -6441,6 +6441,9 @@ static void atomic_switch_perf_msrs(struct vcpu_vmx *vmx)
>>> int i, nr_msrs;
>>> struct perf_guest_switch_msr *msrs;
>>>
>>> + if (!vcpu_to_pmu(&vmx->vcpu)->version)
>>> + return;
>>> +
>>> msrs = perf_guest_get_msrs(&nr_msrs);
>>>
>>> if (!msrs)
>> Personally, I'd prefer this to be expressed as
>>
>> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
>> index 40b1e6138cd5..ace92076c90f 100644
>> --- a/arch/x86/kvm/vmx/vmx.c
>> +++ b/arch/x86/kvm/vmx/vmx.c
>> @@ -6567,7 +6567,9 @@ static void vmx_vcpu_run(struct kvm_vcpu *vcpu)
>>
>> pt_guest_enter(vmx);
>>
>> - atomic_switch_perf_msrs(vmx);
>> + if (vcpu_to_pmu(&vmx->vcpu)->version)
We may use 'vmx->vcpu.arch.pmu.version'.
I would vote in favor of adding the "unlikely (vmx->vcpu.arch.pmu.version)"
check to the atomic_switch_perf_msrs(), which follows pt_guest_enter(vmx).
>> + atomic_switch_perf_msrs(vmx);
>> +
> I just hope the beautiful codes before, I testing this version before
> sending out the patch, ~30 cycles can be saved which means that ~2%
> vmexit time, will update in next version. Let's wait Paolo for other
> opinions below.
You may factor the cost of the "pmu-> version check' itself (~10 cycles)
into your overall 'micro-optimize' revenue.
Thanks,
Like Xu
>
> Wanpeng
>
>> Also, (not knowing much about PMU), is
>> "vcpu_to_pmu(&vmx->vcpu)->version" check correct?
>>
>> E.g. in intel_is_valid_msr() correct for Intel PMU or is it stated
>> somewhere that it is generic rule?
>>
>> Also, speaking about cloud providers and the 'micro' nature of this
>> optimization, would it rather make sense to introduce a static branch
>> (the policy to disable vPMU is likely to be host wide, right)?
>>
>> --
>> Vitaly
>>
On Fri, 13 Mar 2020 at 11:23, Xu, Like <[email protected]> wrote:
>
> Hi Wanpeng,
>
> On 2020/3/12 19:05, Wanpeng Li wrote:
> > On Thu, 12 Mar 2020 at 18:36, Vitaly Kuznetsov <[email protected]> wrote:
> >> Wanpeng Li <[email protected]> writes:
> >>
> >>> From: Wanpeng Li <[email protected]>
> >>>
> >>> PMU is not exposed to guest by most of cloud providers since the bad performance
> >>> of PMU emulation and security concern. However, it calls perf_guest_switch_get_msrs()
> >>> and clear_atomic_switch_msr() unconditionally even if PMU is not exposed to the
> >>> guest before each vmentry.
> >>>
> >>> ~1.28% vmexit time reduced can be observed by kvm-unit-tests/vmexit.flat on my
> >>> SKX server.
> >>>
> >>> Before patch:
> >>> vmcall 1559
> >>>
> >>> After patch:
> >>> vmcall 1539
> >>>
> >>> Signed-off-by: Wanpeng Li <[email protected]>
> >>> ---
> >>> arch/x86/kvm/vmx/vmx.c | 3 +++
> >>> 1 file changed, 3 insertions(+)
> >>>
> >>> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> >>> index 40b1e61..fd526c8 100644
> >>> --- a/arch/x86/kvm/vmx/vmx.c
> >>> +++ b/arch/x86/kvm/vmx/vmx.c
> >>> @@ -6441,6 +6441,9 @@ static void atomic_switch_perf_msrs(struct vcpu_vmx *vmx)
> >>> int i, nr_msrs;
> >>> struct perf_guest_switch_msr *msrs;
> >>>
> >>> + if (!vcpu_to_pmu(&vmx->vcpu)->version)
> >>> + return;
> >>> +
> >>> msrs = perf_guest_get_msrs(&nr_msrs);
> >>>
> >>> if (!msrs)
> >> Personally, I'd prefer this to be expressed as
> >>
> >> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> >> index 40b1e6138cd5..ace92076c90f 100644
> >> --- a/arch/x86/kvm/vmx/vmx.c
> >> +++ b/arch/x86/kvm/vmx/vmx.c
> >> @@ -6567,7 +6567,9 @@ static void vmx_vcpu_run(struct kvm_vcpu *vcpu)
> >>
> >> pt_guest_enter(vmx);
> >>
> >> - atomic_switch_perf_msrs(vmx);
> >> + if (vcpu_to_pmu(&vmx->vcpu)->version)
> We may use 'vmx->vcpu.arch.pmu.version'.
Thanks for confirm this. Maybe this is better:
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 40b1e61..b20423c 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -6567,7 +6567,8 @@ static void vmx_vcpu_run(struct kvm_vcpu *vcpu)
pt_guest_enter(vmx);
- atomic_switch_perf_msrs(vmx);
+ if (vcpu_to_pmu(vcpu)->version)
+ atomic_switch_perf_msrs(vmx);
atomic_switch_umwait_control_msr(vmx);
if (enable_preemption_timer)
>
> I would vote in favor of adding the "unlikely (vmx->vcpu.arch.pmu.version)"
> check to the atomic_switch_perf_msrs(), which follows pt_guest_enter(vmx).
This is hotpath, let's save the cost of function call.
Wanpeng
>
> >> + atomic_switch_perf_msrs(vmx);
> >> +
> > I just hope the beautiful codes before, I testing this version before
> > sending out the patch, ~30 cycles can be saved which means that ~2%
> > vmexit time, will update in next version. Let's wait Paolo for other
> > opinions below.
>
> You may factor the cost of the "pmu-> version check' itself (~10 cycles)
> into your overall 'micro-optimize' revenue.
>
> Thanks,
> Like Xu
> >
> > Wanpeng
> >
> >> Also, (not knowing much about PMU), is
> >> "vcpu_to_pmu(&vmx->vcpu)->version" check correct?
> >>
> >> E.g. in intel_is_valid_msr() correct for Intel PMU or is it stated
> >> somewhere that it is generic rule?
> >>
> >> Also, speaking about cloud providers and the 'micro' nature of this
> >> optimization, would it rather make sense to introduce a static branch
> >> (the policy to disable vPMU is likely to be host wide, right)?
> >>
> >> --
> >> Vitaly
> >>
>
On 2020/3/13 11:39, Wanpeng Li wrote:
> On Fri, 13 Mar 2020 at 11:23, Xu, Like <[email protected]> wrote:
>>
>> Hi Wanpeng,
>>
>> On 2020/3/12 19:05, Wanpeng Li wrote:
>>> On Thu, 12 Mar 2020 at 18:36, Vitaly Kuznetsov <[email protected]> wrote:
>>>> Wanpeng Li <[email protected]> writes:
>>>>
>>>>> From: Wanpeng Li <[email protected]>
>>>>>
>>>>> PMU is not exposed to guest by most of cloud providers since the bad performance
>>>>> of PMU emulation and security concern. However, it calls perf_guest_switch_get_msrs()
>>>>> and clear_atomic_switch_msr() unconditionally even if PMU is not exposed to the
>>>>> guest before each vmentry.
>>>>>
>>>>> ~1.28% vmexit time reduced can be observed by kvm-unit-tests/vmexit.flat on my
>>>>> SKX server.
>>>>>
>>>>> Before patch:
>>>>> vmcall 1559
>>>>>
>>>>> After patch:
>>>>> vmcall 1539
>>>>>
>>>>> Signed-off-by: Wanpeng Li <[email protected]>
>>>>> ---
>>>>> arch/x86/kvm/vmx/vmx.c | 3 +++
>>>>> 1 file changed, 3 insertions(+)
>>>>>
>>>>> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
>>>>> index 40b1e61..fd526c8 100644
>>>>> --- a/arch/x86/kvm/vmx/vmx.c
>>>>> +++ b/arch/x86/kvm/vmx/vmx.c
>>>>> @@ -6441,6 +6441,9 @@ static void atomic_switch_perf_msrs(struct vcpu_vmx *vmx)
>>>>> int i, nr_msrs;
>>>>> struct perf_guest_switch_msr *msrs;
>>>>>
>>>>> + if (!vcpu_to_pmu(&vmx->vcpu)->version)
>>>>> + return;
>>>>> +
>>>>> msrs = perf_guest_get_msrs(&nr_msrs);
>>>>>
>>>>> if (!msrs)
>>>> Personally, I'd prefer this to be expressed as
>>>>
>>>> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
>>>> index 40b1e6138cd5..ace92076c90f 100644
>>>> --- a/arch/x86/kvm/vmx/vmx.c
>>>> +++ b/arch/x86/kvm/vmx/vmx.c
>>>> @@ -6567,7 +6567,9 @@ static void vmx_vcpu_run(struct kvm_vcpu *vcpu)
>>>>
>>>> pt_guest_enter(vmx);
>>>>
>>>> - atomic_switch_perf_msrs(vmx);
>>>> + if (vcpu_to_pmu(&vmx->vcpu)->version)
>> We may use 'vmx->vcpu.arch.pmu.version'.
>
> Thanks for confirm this. Maybe this is better:
>
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 40b1e61..b20423c 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -6567,7 +6567,8 @@ static void vmx_vcpu_run(struct kvm_vcpu *vcpu)
>
> pt_guest_enter(vmx);
>
> - atomic_switch_perf_msrs(vmx);
> + if (vcpu_to_pmu(vcpu)->version)
> + atomic_switch_perf_msrs(vmx);
> atomic_switch_umwait_control_msr(vmx);
>
> if (enable_preemption_timer)
>
>>
>> I would vote in favor of adding the "unlikely (vmx->vcpu.arch.pmu.version)"
>> check to the atomic_switch_perf_msrs(), which follows pt_guest_enter(vmx).
>
> This is hotpath, let's save the cost of function call.
You're right, I measured both.
We may fix pt_guest_enter() with static_branch_unlikely
for a little bit more micro-optimize as well.
Thanks,
Like Xu
>
> Wanpeng
>
>>
>>>> + atomic_switch_perf_msrs(vmx);
>>>> +
>>> I just hope the beautiful codes before, I testing this version before
>>> sending out the patch, ~30 cycles can be saved which means that ~2%
>>> vmexit time, will update in next version. Let's wait Paolo for other
>>> opinions below.
>>
>> You may factor the cost of the "pmu-> version check' itself (~10 cycles)
>> into your overall 'micro-optimize' revenue.
>>
>> Thanks,
>> Like Xu
>>>
>>> Wanpeng
>>>
>>>> Also, (not knowing much about PMU), is
>>>> "vcpu_to_pmu(&vmx->vcpu)->version" check correct?
>>>>
>>>> E.g. in intel_is_valid_msr() correct for Intel PMU or is it stated
>>>> somewhere that it is generic rule?
>>>>
>>>> Also, speaking about cloud providers and the 'micro' nature of this
>>>> optimization, would it rather make sense to introduce a static branch
>>>> (the policy to disable vPMU is likely to be host wide, right)?
>>>>
>>>> --
>>>> Vitaly
>>>>
>>
Jim Mattson <[email protected]> writes:
> On Thu, Mar 12, 2020 at 3:36 AM Vitaly Kuznetsov <[email protected]> wrote:
>
>> Also, speaking about cloud providers and the 'micro' nature of this
>> optimization, would it rather make sense to introduce a static branch
>> (the policy to disable vPMU is likely to be host wide, right)?
>
> Speaking for a cloud provider, no, the policy is not likely to be host-wide.
Ah, then it's just my flawed picture of the world where hosts only run
instances of the same type/family because it's mych easier to partition
them this way.
Scratch the static branch idea then.
--
Vitaly