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 14:44:25 +0800 |
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On 5/21/2020 2:37 PM, Xiaoyao Li wrote: > 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?
I am sorry, I mean: By default, we don't expose WAITPKG to guest. For QEMU, we can use "-overcommit cpu-pm=on" to use WAITPKG.
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