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

[Like Xu <like.xu.linux@gmail.com>]

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
+		 * 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.
+		 */
+		if (edx.split.num_counters_fixed < 4 &&
+		    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