linux-kernel - Re: [PATCH v2] KVM: SVM: let alternatives handle the cases when RSB filling is required

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Date: Fri, 28 Jun 2024 09:09:15 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Amit Shah <amit@...nel.org>
Cc: pbonzini@...hat.com, x86@...nel.org, kvm@...r.kernel.org, 
	linux-kernel@...r.kernel.org, amit.shah@....com, tglx@...utronix.de, 
	mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com, hpa@...or.com, 
	kim.phillips@....com, david.kaplan@....com
Subject: Re: [PATCH v2] KVM: SVM: let alternatives handle the cases when RSB
 filling is required

On Wed, Jun 26, 2024, Amit Shah wrote:
> ---
>  arch/x86/kvm/svm/vmenter.S | 8 ++------
>  1 file changed, 2 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/vmenter.S b/arch/x86/kvm/svm/vmenter.S
> index a0c8eb37d3e1..2ed80aea3bb1 100644
> --- a/arch/x86/kvm/svm/vmenter.S
> +++ b/arch/x86/kvm/svm/vmenter.S
> @@ -209,10 +209,8 @@ SYM_FUNC_START(__svm_vcpu_run)
>  7:	vmload %_ASM_AX
>  8:
>  
> -#ifdef CONFIG_MITIGATION_RETPOLINE
>  	/* IMPORTANT: Stuff the RSB immediately after VM-Exit, before RET! */
> -	FILL_RETURN_BUFFER %_ASM_AX, RSB_CLEAR_LOOPS, X86_FEATURE_RETPOLINE
> -#endif
> +	FILL_RETURN_BUFFER %_ASM_AX, RSB_CLEAR_LOOPS, X86_FEATURE_RSB_VMEXIT

Out of an abundance of paranoia, shouldn't this be?

	FILL_RETURN_BUFFER %_ASM_CX, RSB_CLEAR_LOOPS, X86_FEATURE_RSB_VMEXIT,\
			   X86_FEATURE_RSB_VMEXIT_LITE

Hmm, but it looks like that would incorrectly trigger the "lite" flavor for
families 0xf - 0x12.  I assume those old CPUs aren't affected by whatever on earth
EIBRS_PBRSB is.

	/* AMD Family 0xf - 0x12 */
	VULNWL_AMD(0x0f,	NO_MELTDOWN | NO_SSB | NO_L1TF | NO_MDS | NO_SWAPGS | NO_ITLB_MULTIHIT | NO_MMIO | NO_BHI),
	VULNWL_AMD(0x10,	NO_MELTDOWN | NO_SSB | NO_L1TF | NO_MDS | NO_SWAPGS | NO_ITLB_MULTIHIT | NO_MMIO | NO_BHI),
	VULNWL_AMD(0x11,	NO_MELTDOWN | NO_SSB | NO_L1TF | NO_MDS | NO_SWAPGS | NO_ITLB_MULTIHIT | NO_MMIO | NO_BHI),
	VULNWL_AMD(0x12,	NO_MELTDOWN | NO_SSB | NO_L1TF | NO_MDS | NO_SWAPGS | NO_ITLB_MULTIHIT | NO_MMIO | NO_BHI),

	/* FAMILY_ANY must be last, otherwise 0x0f - 0x12 matches won't work */
	VULNWL_AMD(X86_FAMILY_ANY,	NO_MELTDOWN | NO_L1TF | NO_MDS | NO_SWAPGS | NO_ITLB_MULTIHIT | NO_MMIO | NO_EIBRS_PBRSB | NO_BHI),
	VULNWL_HYGON(X86_FAMILY_ANY,	NO_MELTDOWN | NO_L1TF | NO_MDS | NO_SWAPGS | NO_ITLB_MULTIHIT | NO_MMIO | NO_EIBRS_PBRSB | NO_BHI),

>  
>  	/* Clobbers RAX, RCX, RDX.  */
>  	RESTORE_HOST_SPEC_CTRL
> @@ -348,10 +346,8 @@ SYM_FUNC_START(__svm_sev_es_vcpu_run)
>  
>  2:	cli
>  
> -#ifdef CONFIG_MITIGATION_RETPOLINE
>  	/* IMPORTANT: Stuff the RSB immediately after VM-Exit, before RET! */
> -	FILL_RETURN_BUFFER %rax, RSB_CLEAR_LOOPS, X86_FEATURE_RETPOLINE
> -#endif
> +	FILL_RETURN_BUFFER %rax, RSB_CLEAR_LOOPS, X86_FEATURE_RSB_VMEXIT
>  
>  	/* Clobbers RAX, RCX, RDX, consumes RDI (@svm) and RSI (@spec_ctrl_intercepted). */
>  	RESTORE_HOST_SPEC_CTRL
> -- 
> 2.45.2
>