linux-kernel - Re: [RFC PATCH 37/41] KVM: x86/pmu: Allow writing to fixed counter selector if counter is exposed

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Date: Thu, 11 Apr 2024 15:03:17 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Xiong Zhang <xiong.y.zhang@...ux.intel.com>
Cc: pbonzini@...hat.com, peterz@...radead.org, mizhang@...gle.com, 
	kan.liang@...el.com, zhenyuw@...ux.intel.com, dapeng1.mi@...ux.intel.com, 
	jmattson@...gle.com, kvm@...r.kernel.org, linux-perf-users@...r.kernel.org, 
	linux-kernel@...r.kernel.org, zhiyuan.lv@...el.com, eranian@...gle.com, 
	irogers@...gle.com, samantha.alt@...el.com, like.xu.linux@...il.com, 
	chao.gao@...el.com
Subject: Re: [RFC PATCH 37/41] KVM: x86/pmu: Allow writing to fixed counter
 selector if counter is exposed

On Fri, Jan 26, 2024, Xiong Zhang wrote:
> From: Mingwei Zhang <mizhang@...gle.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@...gle.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.

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)