linux-kernel - Re: [PATCH v2 23/23] kvm: x86: Disable RDMSR interception of IA32_XFD

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Message-ID: <e6fd3fc5-ea06-30a5-29ce-1ffd6b815b47@redhat.com>
Date:   Mon, 20 Dec 2021 10:08:49 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Jing Liu <jing2.liu@...el.com>, x86@...nel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com
Cc:     seanjc@...gle.com, jun.nakajima@...el.com, kevin.tian@...el.com,
        jing2.liu@...ux.intel.com, guang.zeng@...el.com,
        wei.w.wang@...el.com, yang.zhong@...el.com
Subject: Re: [PATCH v2 23/23] kvm: x86: Disable RDMSR interception of
 IA32_XFD_ERR

On 12/17/21 16:30, Jing Liu wrote:
> Also disable read emulation of IA32_XFD_ERR MSR at the same point
> where r/w emulation of IA32_XFD MSR is disabled. 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.
> 
> Signed-off-by: Jing Liu <jing2.liu@...el.com>

Why not do this unconditionally (i.e. in patch 13, with the call to 
vmx_disable_intercept_for_msr in function vmx_create_vcpu)?

Thanks,

Paolo

> ---
>   arch/x86/kvm/vmx/vmx.c | 2 ++
>   arch/x86/kvm/vmx/vmx.h | 2 +-
>   2 files changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 97a823a3f23f..b66a005f076b 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -163,6 +163,7 @@ static u32 vmx_possible_passthrough_msrs[MAX_POSSIBLE_PASSTHROUGH_MSRS] = {
>   	MSR_GS_BASE,
>   	MSR_KERNEL_GS_BASE,
>   	MSR_IA32_XFD,
> +	MSR_IA32_XFD_ERR,
>   #endif
>   	MSR_IA32_SYSENTER_CS,
>   	MSR_IA32_SYSENTER_ESP,
> @@ -1934,6 +1935,7 @@ static u64 vcpu_supported_debugctl(struct kvm_vcpu *vcpu)
>   static void vmx_set_xfd_passthrough(struct kvm_vcpu *vcpu)
>   {
>   	vmx_disable_intercept_for_msr(vcpu, MSR_IA32_XFD, MSR_TYPE_RW);
> +	vmx_disable_intercept_for_msr(vcpu, MSR_IA32_XFD_ERR, MSR_TYPE_R);
>   	vcpu->arch.xfd_out_of_sync = true;
>   }
>   #endif
> diff --git a/arch/x86/kvm/vmx/vmx.h b/arch/x86/kvm/vmx/vmx.h
> index bf9d3051cd6c..0a00242a91e7 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	14
> +#define MAX_POSSIBLE_PASSTHROUGH_MSRS	15
>   	struct {
>   		DECLARE_BITMAP(read, MAX_POSSIBLE_PASSTHROUGH_MSRS);
>   		DECLARE_BITMAP(write, MAX_POSSIBLE_PASSTHROUGH_MSRS);
>