"Shared MSRs" are guest MSRs that are written to the host MSRs but keep their value until the next return to userspace. They support a mask, so that some bits keep the host value, but this mask is only used to skip an unnecessary MSR write and the value written to the MSR is always the guest MSR.
Fix this and, while at it, do not update smsr->values[slot].curr if for whatever reason the wrmsr fails. This should only happen due to reserved bits, so the value written to smsr->values[slot].curr will not match when the user-return notifier and the host value will always be restored. However, it is untidy and in rare cases this can actually avoid spurious WRMSRs on return to userspace.
Cc: stable@vger.kernel.org Signed-off-by: Paolo Bonzini pbonzini@redhat.com --- arch/x86/kvm/x86.c | 5 +++-- 1 file changed, 3 insertions(+), 2 deletions(-)
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index 6ea735d632e9..02863998af91 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -300,13 +300,14 @@ int kvm_set_shared_msr(unsigned slot, u64 value, u64 mask) struct kvm_shared_msrs *smsr = per_cpu_ptr(shared_msrs, cpu); int err;
- if (((value ^ smsr->values[slot].curr) & mask) == 0) + value = (value & mask) | (smsr->values[slot].host & ~mask); + if (value == smsr->values[slot].curr) return 0; - smsr->values[slot].curr = value; err = wrmsrl_safe(shared_msrs_global.msrs[slot], value); if (err) return 1;
+ smsr->values[slot].curr = value; if (!smsr->registered) { smsr->urn.on_user_return = kvm_on_user_return; user_return_notifier_register(&smsr->urn);