linux-kernel - Re: [PATCH v4 14/21] kvm: x86: Disable RDMSR interception of IA32_XFD

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Date:   Tue, 4 Jan 2022 19:34:36 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Yang Zhong <yang.zhong@...el.com>
Cc:     x86@...nel.org, kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-kselftest@...r.kernel.org,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, pbonzini@...hat.com, corbet@....net,
        shuah@...nel.org, jun.nakajima@...el.com, kevin.tian@...el.com,
        jing2.liu@...ux.intel.com, jing2.liu@...el.com,
        guang.zeng@...el.com, wei.w.wang@...el.com
Subject: Re: [PATCH v4 14/21] kvm: x86: Disable RDMSR interception of
 IA32_XFD_ERR

On Wed, Dec 29, 2021, Yang Zhong wrote:
> From: Jing Liu <jing2.liu@...el.com>
> 
> Disable read emulation of IA32_XFD_ERR MSR if guest cpuid includes XFD.
> This saves one unnecessary VM-exit in guest #NM handler, given that the
> MSR is already restored with the guest value before the guest is resumed.
> 
> Suggested-by: Paolo Bonzini <pbonzini@...hat.com>
> Signed-off-by: Jing Liu <jing2.liu@...el.com>
> Signed-off-by: Yang Zhong <yang.zhong@...el.com>
> ---
>  arch/x86/kvm/vmx/vmx.c | 6 ++++++
>  arch/x86/kvm/vmx/vmx.h | 2 +-
>  2 files changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 4e51de876085..638665b3e241 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -162,6 +162,7 @@ static u32 vmx_possible_passthrough_msrs[MAX_POSSIBLE_PASSTHROUGH_MSRS] = {
>  	MSR_FS_BASE,
>  	MSR_GS_BASE,
>  	MSR_KERNEL_GS_BASE,
> +	MSR_IA32_XFD_ERR,
>  #endif
>  	MSR_IA32_SYSENTER_CS,
>  	MSR_IA32_SYSENTER_ESP,
> @@ -7228,6 +7229,11 @@ static void vmx_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
>  		}
>  	}
>  
> +	if (boot_cpu_has(X86_FEATURE_XFD))

This should be kvm_cpu_cap_has(), not boot_cpu_has().  If 32-bit kernels don't
suppress XFD in boot_cpu_data, then using boot_cpus_has() is wrong.  And even if
XFD is suppressed, using kvm_cpu_cap_has() is still preferable.

> +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_XFD_ERR, MSR_TYPE_R,
> +					  !guest_cpuid_has(vcpu, X86_FEATURE_XFD));
> +
> +
>  	set_cr4_guest_host_mask(vmx);
>  
>  	vmx_write_encls_bitmap(vcpu, NULL);
> diff --git a/arch/x86/kvm/vmx/vmx.h b/arch/x86/kvm/vmx/vmx.h
> index 4df2ac24ffc1..bf9d3051cd6c 100644
> --- a/arch/x86/kvm/vmx/vmx.h
> +++ b/arch/x86/kvm/vmx/vmx.h
> @@ -340,7 +340,7 @@ struct vcpu_vmx {
>  	struct lbr_desc lbr_desc;
>  
>  	/* Save desired MSR intercept (read: pass-through) state */
> -#define MAX_POSSIBLE_PASSTHROUGH_MSRS	13
> +#define MAX_POSSIBLE_PASSTHROUGH_MSRS	14
>  	struct {
>  		DECLARE_BITMAP(read, MAX_POSSIBLE_PASSTHROUGH_MSRS);
>  		DECLARE_BITMAP(write, MAX_POSSIBLE_PASSTHROUGH_MSRS);