linux-kernel - Re: [PATCH] kvm: x86: limit the maximum number of vPMU fixed counters to 3

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Date:   Wed, 8 Jul 2020 15:51:05 +0800
From:   "Xu, Like" <like.xu@...el.com>
To:     Like Xu <like.xu@...ux.intel.com>,
        Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org
Cc:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] kvm: x86: limit the maximum number of vPMU fixed counters
 to 3

Kindly ping.

I think we may need this patch, as we limit the maximum vPMU version to 2:
     eax.split.version_id = min(cap.version, 2);

Thanks,
Like Xu

On 2020/6/24 9:59, Like Xu wrote:
> Some new Intel platforms (such as TGL) already have the
> fourth fixed counter TOPDOWN.SLOTS, but it has not been
> fully enabled on KVM and the host.
>
> Therefore, we limit edx.split.num_counters_fixed to 3,
> so that it does not break the kvm-unit-tests PMU test
> case and bad-handled userspace.
>
> Signed-off-by: Like Xu <like.xu@...ux.intel.com>
> ---
>   arch/x86/kvm/cpuid.c | 2 +-
>   arch/x86/kvm/pmu.h   | 2 ++
>   2 files changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 8a294f9747aa..0a2c6d2b4650 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -604,7 +604,7 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function)
>   		eax.split.bit_width = cap.bit_width_gp;
>   		eax.split.mask_length = cap.events_mask_len;
>   
> -		edx.split.num_counters_fixed = cap.num_counters_fixed;
> +		edx.split.num_counters_fixed = min(cap.num_counters_fixed, MAX_FIXED_COUNTERS);
>   		edx.split.bit_width_fixed = cap.bit_width_fixed;
>   		edx.split.reserved = 0;
>   
> diff --git a/arch/x86/kvm/pmu.h b/arch/x86/kvm/pmu.h
> index ab85eed8a6cc..067fef51760c 100644
> --- a/arch/x86/kvm/pmu.h
> +++ b/arch/x86/kvm/pmu.h
> @@ -15,6 +15,8 @@
>   #define VMWARE_BACKDOOR_PMC_REAL_TIME		0x10001
>   #define VMWARE_BACKDOOR_PMC_APPARENT_TIME	0x10002
>   
> +#define MAX_FIXED_COUNTERS	3
> +
>   struct kvm_event_hw_type_mapping {
>   	u8 eventsel;
>   	u8 unit_mask;