On Wed, Jan 05, 2022, Like Xu wrote:
> From: Like Xu <likexu@xxxxxxxxxxx>
>
> 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@xxxxxxxxxxx>
> ---
> 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
>
