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SubjectRe: [RFC PATCH 37/41] KVM: x86/pmu: Allow writing to fixed counter selector if counter is exposed
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On 4/12/2024 6:03 AM, Sean Christopherson wrote:
> On Fri, Jan 26, 2024, Xiong Zhang wrote:
>> From: Mingwei Zhang <mizhang@google.com>
>>
>> Allow writing to fixed counter selector if counter is exposed. If this
>> fixed counter is filtered out, this counter won't be enabled on HW.
>>
>> Passthrough PMU implements the context switch at VM Enter/Exit boundary the
>> guest value cannot be directly written to HW since the HW PMU is owned by
>> the host. Introduce a new field fixed_ctr_ctrl_hw in kvm_pmu to cache the
>> guest value. which will be assigne to HW at PMU context restore.
>>
>> Since passthrough PMU intercept writes to fixed counter selector, there is
>> no need to read the value at pmu context save, but still clear the fix
>> counter ctrl MSR and counters when switching out to host PMU.
>>
>> Signed-off-by: Mingwei Zhang <mizhang@google.com>
>> ---
>> arch/x86/include/asm/kvm_host.h | 1 +
>> arch/x86/kvm/vmx/pmu_intel.c | 28 ++++++++++++++++++++++++----
>> 2 files changed, 25 insertions(+), 4 deletions(-)
>>
>> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
>> index fd1c69371dbf..b02688ed74f7 100644
>> --- a/arch/x86/include/asm/kvm_host.h
>> +++ b/arch/x86/include/asm/kvm_host.h
>> @@ -527,6 +527,7 @@ struct kvm_pmu {
>> unsigned nr_arch_fixed_counters;
>> unsigned available_event_types;
>> u64 fixed_ctr_ctrl;
>> + u64 fixed_ctr_ctrl_hw;
>> u64 fixed_ctr_ctrl_mask;
> Before introduce more fields, can someone please send a patch/series to rename
> the _mask fields? AFAIK, they all should be e.g. fixed_ctr_ctrl_rsvd, or something
> to that effect.

Yeah, I remember I ever said to cook a patch to rename all these _mask
fields. I would do it now.


>
> Because I think we should avoid reinventing the naming wheel, and use "shadow"
> instead of "hw", because KVM developers already know what "shadow" means. But
> "mask" also has very specific meaning for shadowed fields. That, and "mask" is
> a freaking awful name in the first place.
>
>> u64 global_ctrl;
>> u64 global_status;
>> diff --git a/arch/x86/kvm/vmx/pmu_intel.c b/arch/x86/kvm/vmx/pmu_intel.c
>> index 713c2a7c7f07..93cfb86c1292 100644
>> --- a/arch/x86/kvm/vmx/pmu_intel.c
>> +++ b/arch/x86/kvm/vmx/pmu_intel.c
>> @@ -68,6 +68,25 @@ static int fixed_pmc_events[] = {
>> [2] = PSEUDO_ARCH_REFERENCE_CYCLES,
>> };
>>
>> +static void reprogram_fixed_counters_in_passthrough_pmu(struct kvm_pmu *pmu, u64 data)
> We need to come up with shorter names, this ain't Java. :-) Heh, that can be
> another argument for "mediated", it saves three characters.
>
> And somewhat related, kernel style is <scope>_<blah>, i.e.
>
> static void mediated_pmu_reprogram_fixed_counters(struct kvm_pmu *pmu, u64 data)

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