KVM: x86: nSVM: disallow userspace setting of MSR_AMD64_TSC_RATIO to non default...

If nested tsc scaling is disabled, MSR_AMD64_TSC_RATIO should
never have non default value.

Due to way nested tsc scaling support was implmented in qemu,
it would set this msr to 0 when nested tsc scaling was disabled.
Ignore that value for now, as it causes no harm.

Fixes: 5228eb96a487 ("KVM: x86: nSVM: implement nested TSC scaling")
Cc: stable@vger.kernel.org
Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
Message-Id: <20220223115649.319134-1-mlevitsk@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 821edf6..fd3a00c 100644 (file)
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -2693,8 +2693,23 @@ static int svm_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr)
        u64 data = msr->data;
        switch (ecx) {
        case MSR_AMD64_TSC_RATIO:
-               if (!msr->host_initiated && !svm->tsc_scaling_enabled)
-                       return 1;
+
+               if (!svm->tsc_scaling_enabled) {
+
+                       if (!msr->host_initiated)
+                               return 1;
+                       /*
+                        * In case TSC scaling is not enabled, always
+                        * leave this MSR at the default value.
+                        *
+                        * Due to bug in qemu 6.2.0, it would try to set
+                        * this msr to 0 if tsc scaling is not enabled.
+                        * Ignore this value as well.
+                        */
+                       if (data != 0 && data != svm->tsc_ratio_msr)
+                               return 1;
+                       break;
+               }
 
                if (data & TSC_RATIO_RSVD)
                        return 1;