Messages in this thread | | | Subject | Re: [PATCH 2/2] kvm/x86: don't expose MSR_IA32_UMWAIT_CONTROL unconditionally | From | Xiaoyao Li <> | Date | Thu, 21 May 2020 14:37:17 +0800 |
| |
On 5/21/2020 1:28 PM, Tao Xu wrote: > > > 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.
I guess you typed "unconditionally" by mistake that you meant to say "conditionally" in fact?
| |