Re: [PATCH] kvm/x86/vmx: switch MSR_MISC_FEATURES_ENABLES between host and guest

[Kyle Huey]

On Thu, Mar 14, 2019 at 7:50 PM Xiaoyao Li <xiaoyao.li@xxxxxxxxxxxxxxx> wrote:
>
> CPUID Faulting is a feature about CPUID instruction. When CPUID Faulting is
> enabled, all execution of the CPUID instruction outside system-management
> mode (SMM) cause a general-protection (#GP) if the CPL > 0.
>
> About this feature, detailed information can be found at
> https://www.intel.com/content/dam/www/public/us/en/documents/application-notes/virtualization-technology-flexmigration-application-note.pdf
>
> There is an issue that current kvm doesn't switch the value of
> MSR_MISC_FEATURES_ENABLES between host and guest. If MSR_MISC_FEATURES_ENABLES
> exists on the hardware cpu, and host enables CPUID faulting (setting the bit 0
> of MSR_MISC_FEATURES_ENABLES), it will impact the guest's behavior because
> cpuid faulting is enabled by host and passed to guest.

The host doesn't enable CPUID faulting and keep it enabled for
everything though, it only enables it for specified user-space
processes (via arch_prctl). How does CPUID faulting "leak" into the
KVM guest?

- Kyle

> From my tests, when host enables cpuid faulting, it causes guest boot failure
> when guest uses *modprobe* to load modules. Below is the error log:
>
> [    1.233556] traps: modprobe[71] general protection fault ip:7f0077f6495c sp:7ffda148d808 error:0 in ld-2.17.so[7f0077f4d000+22000]
> [    1.237780] traps: modprobe[73] general protection fault ip:7fad5aba095c sp:7ffd36067378 error:0 in ld-2.17.so[7fad5ab89000+22000]
> [    1.241930] traps: modprobe[75] general protection fault ip:7f3edb89495c sp:7fffa1a81308 error:0 in ld-2.17.so[7f3edb87d000+22000]
> [    1.245998] traps: modprobe[77] general protection fault ip:7f91d670895c sp:7ffc25fa7f38 error:0 in ld-2.17.so[7f91d66f1000+22000]
> [    1.250016] traps: modprobe[79] general protection fault ip:7f0ddbbdc95c sp:7ffe9c34f8d8 error:0 in ld-2.17.so[7f0ddbbc5000+22000]
>
> *modprobe* calls CPUID instruction thus causing cpuid faulting in guest.
> At the end, because guest cannot *modprobe* modules, it boots failure.
>
> This patch switches MSR_MISC_FEATURES_ENABLES between host and guest when
> hardware has this MSR.
>
> This patch doesn't confict with the commit db2336a80489 ("KVM: x86: virtualize
> cpuid faulting"), which provides a software emulation of cpuid faulting for
> x86 arch. Below analysing how cpuid faulting will work after applying this patch:
>
> 1. If host cpu is AMD. It doesn't have MSR_MISC_FEATURES_ENABLES, so we can just
> use the software emulation of cpuid faulting.
>
> 2. If host cpu is Intel and it doesn't have MSR_MISC_FEATURES_ENABLES. The same
> as case 1, we can just use the software emulation of cpuid faulting.
>
> 3. If host cpu is Intel and it has MSR_MISC_FEATURES_ENABLES. With this patch,
> it will write guest's value into MSR_MISC_FEATURES_ENABLES when vm entry.
> If guest enables cpuid faulting and when guest calls CPUID instruction with
> CPL > 0, it will cause #GP exception in guest instead of VM exit because of
> CPUID, thus it doesn't go to the kvm emualtion path but ues the hardware
> feature. Also it's a benefit that we needn't use VM exit to inject #GP to
> emulate cpuid faulting feature.
>
> Intel SDM vol3.25.1.1 specifies the priority between cpuid faulting
> and CPUID instruction.
>
> Signed-off-by: Xiaoyao Li <xiaoyao.li@xxxxxxxxxxxxxxx>
> ---
>  arch/x86/kvm/vmx/vmx.c | 19 +++++++++++++++++++
>  1 file changed, 19 insertions(+)
>
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 30a6bcd735ec..90707fae688e 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -6321,6 +6321,23 @@ static void atomic_switch_perf_msrs(struct vcpu_vmx *vmx)
>                                         msrs[i].host, false);
>  }
>
> +static void atomic_switch_msr_misc_features_enables(struct kvm_vcpu *vcpu)
> +{
> +       u64 host_msr;
> +       struct vcpu_vmx *vmx = to_vmx(vcpu);
> +
> +       /* if MSR MISC_FEATURES_ENABLES doesn't exist on the hardware, do nothing*/
> +       if (rdmsrl_safe(MSR_MISC_FEATURES_ENABLES, &host_msr))
> +               return;
> +
> +       if (host_msr == vcpu->arch.msr_misc_features_enables)
> +               clear_atomic_switch_msr(vmx, MSR_MISC_FEATURES_ENABLES);
> +       else
> +               add_atomic_switch_msr(vmx, MSR_MISC_FEATURES_ENABLES,
> +                                     vcpu->arch.msr_misc_features_enables,
> +                                     host_msr, false);
> +}
> +
>  static void vmx_arm_hv_timer(struct vcpu_vmx *vmx, u32 val)
>  {
>         vmcs_write32(VMX_PREEMPTION_TIMER_VALUE, val);
> @@ -6562,6 +6579,8 @@ static void vmx_vcpu_run(struct kvm_vcpu *vcpu)
>
>         atomic_switch_perf_msrs(vmx);
>
> +       atomic_switch_msr_misc_features_enables(vcpu);
> +
>         vmx_update_hv_timer(vcpu);
>
>         /*
> --
> 2.19.1