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

[Sean Christopherson <seanjc@google.com>]

On Wed, Jan 05, 2022, Like Xu wrote:
> From: Like Xu <likexu@tencent.com>
> 
> When we choose to disable the fourth fixed counter TOPDOWN.SLOTS,
> we need to also reduce the length of the 0AH.EBX bit vector, which
> enumerates architecture performance monitoring events, and set
> 0AH.EBX.[bit 7] to 1 if the new value of EAX[31:24] is still > 7.
> 
> Fixes: 2e8cd7a3b8287 ("kvm: x86: limit the maximum number of vPMU fixed counters to 3")
> Signed-off-by: Like Xu <likexu@tencent.com>
> ---
>  arch/x86/kvm/cpuid.c | 15 +++++++++++++++
>  1 file changed, 15 insertions(+)
> 
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 0b920e12bb6d..1f0131145296 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -782,6 +782,21 @@ 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 pre-defined architectural event (Topdown Slots) will be supported
> +		 * if the 4th fixed counter exists && EAX[31:24] > 7 && EBX[7] = 0.

Please wrap at ~80 chars.

> +		 *
> +		 * Currently, KVM needs to set EAX[31:24] < 8 or EBX[7] == 1
> +		 * to make this event unavailable in a consistent way.
> +		 */
> +		if (edx.split.num_counters_fixed < 4) {
> +			if (eax.split.mask_length > 7)
> +				eax.split.mask_length--;

This will break if there's a bit>7 enumerated in EBX (events_mask) that KVM wants
to expose to the guest.  It doesn't cause problems today because bits 31:8 are all
reserved, but that will not always be the case.

We could do

		if (edx.split.num_counters_fixed < 4) {
			if (eax.split.mask_length == 7)
				eax.split.mask_length--;
			else
				cap.events_mask |= BIT_ULL(7);
		}

but I don't see any reason to make this more complex than:

		if (edx.split.num_counters_fixed < 4 &&
		    eax.split.mask_length > 7)
			cap.events_mask |= BIT_ULL(7);

> +			if (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
>