Re: [PATCH v2] KVM: x86/pmu: Make top-down.slots event unavailable in supported leaf

[Sean Christopherson <seanjc@google.com>]

On Thu, Jan 06, 2022, Like Xu wrote:
> From: Like Xu <likexu@tencent.com>
> 
> When we choose to disable the fourth fixed counter TOPDOWN.SLOTS,
> we also need to comply with the specification and set 0AH.EBX.[bit 7]
> to 1 if the guest (e.g. on the ICX) has a value of 0AH.EAX[31:24] > 7.
> 
> Fixes: 2e8cd7a3b8287 ("kvm: x86: limit the maximum number of vPMU fixed counters to 3")
> Signed-off-by: Like Xu <likexu@tencent.com>
> ---
> v1 -> v2 Changelog:
> - Make it simpler to keep forward compatibility; (Sean)
> - Wrap related comment at ~80 chars; (Sean)
> 
> Previous:
> https://lore.kernel.org/kvm/20220105050711.67280-1-likexu@tencent.com/
> 
>  arch/x86/kvm/cpuid.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 0b920e12bb6d..4fe17a537084 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -782,6 +782,18 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function)
>  		eax.split.mask_length = cap.events_mask_len;
>  
>  		edx.split.num_counters_fixed = min(cap.num_counters_fixed, MAX_FIXED_COUNTERS);
> +
> +		/*
> +		 * The 8th Intel architectural event (Topdown Slots) will be supported

Nit, the "8th" part is unnecessary information.  

> +		 * if the 4th fixed counter exists && EAX[31:24] > 7 && EBX[7] = 0.
> +		 *
> +		 * Currently, KVM needs to set EAX[31:24] < 8 or EBX[7] == 1
> +		 * to make this event unavailable in a consistent way.
> +		 */

This comment is now slightly stale.  It also doesn't say why the event is made
unavailable.

> +		if (edx.split.num_counters_fixed < 4 &&

Rereading the changelog and the changelog of the Fixed commit, I don't think KVM
should bother checking num_counters_fixed.  IIUC, cap.events_mask[7] should already
be '1' if there are less than 4 fixed counters in hardware, but at the same time
there's no harm in being a bit overzealous.  That would help simplifiy the comment
as there's no need to explain why num_counters_fixed is checked, e.g. the fact that
Topdown Slots uses the 4th fixed counter is irrelevant with respect to the legality
of setting EBX[7]=1 to hide an unsupported event.

		/*
		 * Hide Intel's Topdown Slots architectural event, it's not yet
		 * supported by KVM.
		 */
		if (eax.split.mask_length > 7)
			cap.events_mask |= BIT_ULL(7);

> +		    eax.split.mask_length > 7)
> +			cap.events_mask |= BIT_ULL(7);
> +
>  		edx.split.bit_width_fixed = cap.bit_width_fixed;
>  		if (cap.version)
>  			edx.split.anythread_deprecated = 1;
> -- 
> 2.33.1
>