Messages in this thread | | | Subject | Re: [PATCH 2/2] kvm/x86: don't expose MSR_IA32_UMWAIT_CONTROL unconditionally | From | Tao Xu <> | Date | Thu, 21 May 2020 13:28:52 +0800 |
| |
On 5/21/2020 12:33 PM, Xiaoyao Li wrote: > On 5/21/2020 5:05 AM, Paolo Bonzini wrote: >> 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@redhat.com> >>> --- >>> 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); > > + Tao > > I remember there is certainly some reason why we don't expose WAITPKG to > guest by default. > > Tao, please help clarify it. > > Thanks, > -Xiaoyao >
Because in VM, umwait and tpause can put a (psysical) CPU into a power saving state. So from host view, this cpu will be 100% usage by VM. Although umwait and tpause just cause short wait(maybe 100 microseconds), we still want to unconditionally expose WAITPKG in VM.
| |