linux-kernel - Re: [PATCH v2 2/4] KVM: VMX: Handle the immediate form of MSR instructions

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Message-ID: <aJJjjGWFL-Ju2Efw@google.com>
Date: Tue, 5 Aug 2025 13:03:24 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: "Xin Li (Intel)" <xin@...or.com>
Cc: linux-kernel@...r.kernel.org, kvm@...r.kernel.org, pbonzini@...hat.com, 
	tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, 
	dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com, 
	chao.gao@...el.com
Subject: Re: [PATCH v2 2/4] KVM: VMX: Handle the immediate form of MSR instructions

On Fri, Aug 01, 2025, Xin Li (Intel) wrote:
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index f19a76d3ca0e..c5d0082cf0a5 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -978,6 +978,7 @@ struct kvm_vcpu_arch {
>  	unsigned long guest_debug_dr7;
>  	u64 msr_platform_info;
>  	u64 msr_misc_features_enables;
> +	u32 cui_rdmsr_imm_reg;

This should be an "int", mostly because that's how KVM tracks it throughout the
various accessors, but also because it'd let us use "-1" for an "invalid" value,
e.g. if we ever want to add sanity checks to the completion callback (I don't
think that's worth doing).

> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index aa157fe5b7b3..c112595dfff9 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -6003,6 +6003,23 @@ static int handle_notify(struct kvm_vcpu *vcpu)
>  	return 1;
>  }
>  
> +static int vmx_get_msr_imm_reg(void)

It's a bit silly, but I think it's worth passing in the @vcpu here.  E.g. if we
ever want to support caching the vmcs.VMX_INSTRUCTION_INFO.  And because it costs
literally nothing (barring a truly stupid compiler).

> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index a1c49bc681c4..fe12aae7089c 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1968,6 +1968,13 @@ static void complete_userspace_rdmsr(struct kvm_vcpu *vcpu)
>  	}
>  }
>  
> +static void complete_userspace_rdmsr_imm(struct kvm_vcpu *vcpu)

No need for this helper, the few lines can be open coded in complete_fast_rdmsr_imm().

> +{
> +	if (!vcpu->run->msr.error)
> +		kvm_register_write(vcpu, vcpu->arch.cui_rdmsr_imm_reg,
> +				   vcpu->run->msr.data);
> +}
> +