Re: [PATCH] KVM: VMX: Nop emulation of MSR_IA32_POWER_CTL

Sean Christopherson
2019-04-15T18:17:05Z

On Mon, Apr 15, 2019 at 06:45:26PM +0300, Liran Alon wrote:
> Since commits 668fffa3f838 ("kvm: better MWAIT emulation for guests”)
> and 4d5422cea3b6 ("KVM: X86: Provide a capability to disable MWAIT intercepts”),
> KVM was modified to allow an admin to configure certain guests to execute
> MONITOR/MWAIT inside guest without being intercepted by host.
>
> This is useful in case admin wishes to allocate a dedicated logical
> processor for each vCPU thread. Thus, making it safe for guest to
> completely control the power-state of the logical processor.
>
> The ability to use this new KVM capability was introduced to QEMU by
> commits 6f131f13e68d ("kvm: support -overcommit cpu-pm=on|off”) and
> 2266d4431132 ("i386/cpu: make -cpu host support monitor/mwait”).
>
> However, exposing MONITOR/MWAIT to a Linux guest may cause it's intel_idle
^^^^
its

English is a wonderful language...

> kernel module to execute c1e_promotion_disable() which will attempt to
> RDMSR/WRMSR from/to MSR_IA32_POWER_CTL to manipulate the "C1E Enable"
> bit. This behaviour was introduced by commit
> 32e9518005c8 ("intel_idle: export both C1 and C1E”).

Technically, I think this is a Qemu bug. KVM reports all zeros for
CPUID_MWAIT_LEAF when userspace queries KVM_GET_SUPPORTED_CPUID and
KVM_GET_EMULATED_CPUID. And I think that's correct/desired, supporting
MONITOR/MWAIT sub-features should be a separate enabling patch set.

Note, there is a virtualization hole regarding MWAIT as KVM can't
intercept MWAIT when executed with unsupported hints/features, but
I don't think that absolves Qemu of wrongdoing.

> Becuase KVM doesn't emulate this MSR, running KVM with ignore_msrs=0
> will cause the above guest behaviour to raise a #GP which will cause
> guest to kernel panic.
>
> Therefore, add support for nop emulation of MSR_IA32_POWER_CTL to
> avoid #GP in guest in this scenario.
>
> Future commits can optimise emulation further by reflecting guest
> MSR changes to host MSR to provide guest with the ability to
> fine-tune the dedicated logical processor power-state.
>
> Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
> Signed-off-by: Liran Alon <liran.alon@oracle.com>
> ---
> arch/x86/kvm/vmx/vmx.c | 6 ++++++
> arch/x86/kvm/vmx/vmx.h | 2 ++
> arch/x86/kvm/x86.c | 1 +
> 3 files changed, 9 insertions(+)
>
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 2634ee8c9dc8..6246d782b746 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -1696,6 +1696,9 @@ static int vmx_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> case MSR_IA32_SYSENTER_ESP:
> msr_info->data = vmcs_readl(GUEST_SYSENTER_ESP);
> break;
> + case MSR_IA32_POWER_CTL:
> + msr_info->data = vmx->msr_ia32_power_ctl;
> + break;
> case MSR_IA32_BNDCFGS:
> if (!kvm_mpx_supported() ||
> (!msr_info->host_initiated &&
> @@ -1826,6 +1829,9 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> case MSR_IA32_SYSENTER_ESP:
> vmcs_writel(GUEST_SYSENTER_ESP, data);
> break;
> + case MSR_IA32_POWER_CTL:
> + vmx->msr_ia32_power_ctl = data;
> + break;
> case MSR_IA32_BNDCFGS:
> if (!kvm_mpx_supported() ||
> (!msr_info->host_initiated &&

If KVM does go the route of advertising MWAIT/MONITOR sub-features, then I
think the MSR needs to be emulated on both Intel and AMD. Glancing through
drivers/idle/intel_idle.c, I don't see anything that prevents it from
successfully probing an "Intel" vCPU that is being emulated on AMD hardware.

> diff --git a/arch/x86/kvm/vmx/vmx.h b/arch/x86/kvm/vmx/vmx.h
> index 99328954c2fc..e9772850a2a1 100644
> --- a/arch/x86/kvm/vmx/vmx.h
> +++ b/arch/x86/kvm/vmx/vmx.h
> @@ -259,6 +259,8 @@ struct vcpu_vmx {
>
> unsigned long host_debugctlmsr;
>
> + u64 msr_ia32_power_ctl;
> +
> /*
> * Only bits masked by msr_ia32_feature_control_valid_bits can be set in
> * msr_ia32_feature_control. FEATURE_CONTROL_LOCKED is always included
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 02c8e095a239..39ee4087f954 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1167,6 +1167,7 @@ static u32 emulated_msrs[] = {
> MSR_PLATFORM_INFO,
> MSR_MISC_FEATURES_ENABLES,
> MSR_AMD64_VIRT_SPEC_CTRL,
> + MSR_IA32_POWER_CTL,
> };
>
> static unsigned num_emulated_msrs;
> --
> 2.20.1
>