linux-kernel - Re: [PATCH v3 2/2] KVM: SVM: Add support for Virtual SPEC

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Date:   Tue, 19 Jan 2021 10:31:05 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Babu Moger <babu.moger@....com>
Cc:     pbonzini@...hat.com, tglx@...utronix.de, mingo@...hat.com,
        bp@...en8.de, fenghua.yu@...el.com, tony.luck@...el.com,
        wanpengli@...cent.com, kvm@...r.kernel.org,
        thomas.lendacky@....com, peterz@...radead.org, joro@...tes.org,
        x86@...nel.org, kyung.min.park@...el.com,
        linux-kernel@...r.kernel.org, krish.sadhukhan@...cle.com,
        hpa@...or.com, mgross@...ux.intel.com, vkuznets@...hat.com,
        kim.phillips@....com, wei.huang2@....com, jmattson@...gle.com
Subject: Re: [PATCH v3 2/2] KVM: SVM: Add support for Virtual SPEC_CTRL

On Fri, Jan 15, 2021, Babu Moger wrote:
> ---
>  arch/x86/include/asm/svm.h |    4 +++-
>  arch/x86/kvm/svm/sev.c     |    4 ++++
>  arch/x86/kvm/svm/svm.c     |   19 +++++++++++++++----
>  3 files changed, 22 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/include/asm/svm.h b/arch/x86/include/asm/svm.h
> index 1c561945b426..772e60efe243 100644
> --- a/arch/x86/include/asm/svm.h
> +++ b/arch/x86/include/asm/svm.h
> @@ -269,7 +269,9 @@ struct vmcb_save_area {
>  	 * SEV-ES guests when referenced through the GHCB or for
>  	 * saving to the host save area.
>  	 */
> -	u8 reserved_7[80];
> +	u8 reserved_7[72];
> +	u32 spec_ctrl;		/* Guest version of SPEC_CTRL at 0x2E0 */
> +	u8 reserved_7b[4];

Don't nested_prepare_vmcb_save() and nested_vmcb_checks() need to be updated to
handle the new field, too?

>  	u32 pkru;
>  	u8 reserved_7a[20];
>  	u64 reserved_8;		/* rax already available at 0x01f8 */
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index c8ffdbc81709..959d6e47bd84 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -546,6 +546,10 @@ static int sev_es_sync_vmsa(struct vcpu_svm *svm)
>  	save->pkru = svm->vcpu.arch.pkru;
>  	save->xss  = svm->vcpu.arch.ia32_xss;
>  
> +	/* Update the guest SPEC_CTRL value in the save area */
> +	if (boot_cpu_has(X86_FEATURE_V_SPEC_CTRL))
> +		save->spec_ctrl = svm->spec_ctrl;

I think this can be dropped if svm->spec_ctrl is unused when V_SPEC_CTRL is
supported (see below).  IIUC, the memcpy() that's just out of sight would do
the propgation to the VMSA.

> +
>  	/*
>  	 * SEV-ES will use a VMSA that is pointed to by the VMCB, not
>  	 * the traditional VMSA that is part of the VMCB. Copy the
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 7ef171790d02..a0cb01a5c8c5 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -1244,6 +1244,9 @@ static void init_vmcb(struct vcpu_svm *svm)
>  
>  	svm_check_invpcid(svm);
>  
> +	if (boot_cpu_has(X86_FEATURE_V_SPEC_CTRL))
> +		save->spec_ctrl = svm->spec_ctrl;
> +
>  	if (kvm_vcpu_apicv_active(&svm->vcpu))
>  		avic_init_vmcb(svm);
>  
> @@ -3789,7 +3792,10 @@ static __no_kcsan fastpath_t svm_vcpu_run(struct kvm_vcpu *vcpu)
>  	 * is no need to worry about the conditional branch over the wrmsr
>  	 * being speculatively taken.
>  	 */
> -	x86_spec_ctrl_set_guest(svm->spec_ctrl, svm->virt_spec_ctrl);
> +	if (static_cpu_has(X86_FEATURE_V_SPEC_CTRL))
> +		svm->vmcb->save.spec_ctrl = svm->spec_ctrl;
> +	else
> +		x86_spec_ctrl_set_guest(svm->spec_ctrl, svm->virt_spec_ctrl);

Can't we avoid functional code in svm_vcpu_run() entirely when V_SPEC_CTRL is
supported?  Make this code a nop, disable interception from time zero, and
read/write the VMBC field in svm_{get,set}_msr().  I.e. don't touch
svm->spec_ctrl if V_SPEC_CTRL is supported.  

	if (!static_cpu_has(X86_FEATURE_V_SPEC_CTRL))
		x86_spec_ctrl_set_guest(svm->spec_ctrl, svm->virt_spec_ctrl);

	svm_vcpu_enter_exit(vcpu, svm);

	if (!static_cpu_has(X86_FEATURE_V_SPEC_CTRL) &&
	    unlikely(!msr_write_intercepted(vcpu, MSR_IA32_SPEC_CTRL)))
		svm->spec_ctrl = native_read_msr(MSR_IA32_SPEC_CTRL);

>  	svm_vcpu_enter_exit(vcpu, svm);
>  
> @@ -3808,13 +3814,18 @@ static __no_kcsan fastpath_t svm_vcpu_run(struct kvm_vcpu *vcpu)
>  	 * If the L02 MSR bitmap does not intercept the MSR, then we need to
>  	 * save it.
>  	 */
> -	if (unlikely(!msr_write_intercepted(vcpu, MSR_IA32_SPEC_CTRL)))
> -		svm->spec_ctrl = native_read_msr(MSR_IA32_SPEC_CTRL);
> +	if (unlikely(!msr_write_intercepted(vcpu, MSR_IA32_SPEC_CTRL))) {
> +		if (static_cpu_has(X86_FEATURE_V_SPEC_CTRL))
> +			svm->spec_ctrl = svm->vmcb->save.spec_ctrl;
> +		else
> +			svm->spec_ctrl = native_read_msr(MSR_IA32_SPEC_CTRL);
> +	}
>  
>  	if (!sev_es_guest(svm->vcpu.kvm))
>  		reload_tss(vcpu);
>  
> -	x86_spec_ctrl_restore_host(svm->spec_ctrl, svm->virt_spec_ctrl);
> +	if (!static_cpu_has(X86_FEATURE_V_SPEC_CTRL))
> +		x86_spec_ctrl_restore_host(svm->spec_ctrl, svm->virt_spec_ctrl);
>  
>  	if (!sev_es_guest(svm->vcpu.kvm)) {
>  		vcpu->arch.cr2 = svm->vmcb->save.cr2;
>