Re: [PATCH 5/9] KVM: x86/pmu: Check CPUID.0AH.ECX consistency

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On 9/1/2023 3:28 PM, Xiong Zhang wrote:
With Arch PMU V5, register CPUID.0AH.ECX indicates Fixed Counter
enumeration. It is a bit mask which enumerates the supported Fixed
counters.
FxCtrl[i]_is_supported := ECX[i] || (EDX[4:0] > i)
where EDX[4:0] is Number of continuous fixed-function performance
counters starting from 0 (if version ID > 1).

Here ECX and EDX[4:0] should satisfy the following consistency:
1. if 1 < pmu_version < 5, ECX == 0;
2. if pmu_version == 5 && edx[4:0] == 0, ECX[bit 0] == 0
3. if pmu_version == 5 && edx[4:0] > 0,
    ecx & ((1 << edx[4:0]) - 1) == (1 << edx[4:0]) -1

Otherwise it is mess to decide whether a fixed counter is supported
or not. i.e. pmu_version = 5, edx[4:0] = 3, ecx = 0x10, it is hard
to decide whether fixed counters 0 ~ 2 are supported or not.

User can call SET_CPUID2 ioctl to set guest CPUID.0AH, this commit
adds a check to guarantee ecx and edx consistency specified by user.

Once user specifies an un-consistency value, KVM can return an
error to user and drop user setting, or correct the un-consistency
data and accept the corrected data, this commit chooses to
return an error to user.

Signed-off-by: Xiong Zhang <xiong.y.zhang@xxxxxxxxx>
---
  arch/x86/kvm/cpuid.c | 27 +++++++++++++++++++++++++++
  1 file changed, 27 insertions(+)

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index e961e9a05847..95dc5e8847e0 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -150,6 +150,33 @@ static int kvm_check_cpuid(struct kvm_vcpu *vcpu,
  			return -EINVAL;
  	}
+ best = cpuid_entry2_find(entries, nent, 0xa,
+				 KVM_CPUID_INDEX_NOT_SIGNIFICANT);
+	if (best && vcpu->kvm->arch.enable_pmu) {
+		union cpuid10_eax eax;
+		union cpuid10_edx   edx;


Remove the redundant space before edx.


+
+		eax.full = best->eax;
+		edx.full = best->edx;
+

We may add SDM quotes as comments here. That makes reader to understand the logic easily.


+		if (eax.split.version_id > 1 &&
+		    eax.split.version_id < 5 &&
+		    best->ecx != 0) {
+			return -EINVAL;
+		} else if (eax.split.version_id >= 5) {
+			int fixed_count = edx.split.num_counters_fixed;
+
+			if (fixed_count == 0 && (best->ecx & 0x1)) {
+				return -EINVAL;
+			} else if (fixed_count > 0) {
+				int low_fixed_mask = (1 << fixed_count) - 1;
+
+				if ((best->ecx & low_fixed_mask) != low_fixed_mask)
+					return -EINVAL;
+			}
+		}
+	}
+
  	/*
  	 * Exposing dynamic xfeatures to the guest requires additional
  	 * enabling in the FPU, e.g. to expand the guest XSAVE state size.



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