On 20/05/20 18:07, Maxim Levitsky wrote:
> This msr is only available when the host supports WAITPKG feature.
>
> This breaks a nested guest, if the L1 hypervisor is set to ignore
> unknown msrs, because the only other safety check that the
> kernel does is that it attempts to read the msr and
> rejects it if it gets an exception.
>
> Fixes: 6e3ba4abce KVM: vmx: Emulate MSR IA32_UMWAIT_CONTROL
>
> Signed-off-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>
> ---
> arch/x86/kvm/x86.c | 4 ++++
> 1 file changed, 4 insertions(+)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index fe3a24fd6b263..9c507b32b1b77 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -5314,6 +5314,10 @@ static void kvm_init_msr_list(void)
> if (msrs_to_save_all[i] - MSR_ARCH_PERFMON_EVENTSEL0 >=
> min(INTEL_PMC_MAX_GENERIC, x86_pmu.num_counters_gp))
> continue;
> + break;
> + case MSR_IA32_UMWAIT_CONTROL:
> + if (!kvm_cpu_cap_has(X86_FEATURE_WAITPKG))
> + continue;
> default:
> break;
> }
The patch is correct, and matches what is done for the other entries of
msrs_to_save_all. However, while looking at it I noticed that
X86_FEATURE_WAITPKG is actually never added, and that is because it was
also not added to the supported CPUID in commit e69e72faa3a0 ("KVM: x86:
Add support for user wait instructions", 2019-09-24), which was before
the kvm_cpu_cap mechanism was added.
So while at it you should also fix that. The right way to do that is to
add a
if (vmx_waitpkg_supported())
kvm_cpu_cap_check_and_set(X86_FEATURE_WAITPKG);
in vmx_set_cpu_caps.
Thanks,
Paolo
