Re: [PATCH v2] KVM: VMX: Move VM-exit RSB stuffing out of line

[Sean Christopherson <seanjc@google.com>]

On Thu, Jun 30, 2022, Jim Mattson wrote:
> RSB-stuffing after VM-exit is only needed for legacy CPUs without
> eIBRS. Move the RSB-stuffing code out of line to avoid the JMP on
> modern CPUs.

The shortlog and this sentence need to be updated, the stuffing code is still
in-line, but the JMP is being dropped.

> Note that CPUs that are subject to SpectreRSB attacks need
> RSB-stuffing on VM-exit whether or not RETPOLINE is in use as a
> SpectreBTB mitigation. However, I am leaving the existing mitigation
> strategy alone.
> 
> Signed-off-by: Jim Mattson <jmattson@google.com>
> ---
>  arch/x86/kvm/vmx/vmenter.S | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx/vmenter.S b/arch/x86/kvm/vmx/vmenter.S
> index 435c187927c4..ea5986b96004 100644
> --- a/arch/x86/kvm/vmx/vmenter.S
> +++ b/arch/x86/kvm/vmx/vmenter.S
> @@ -76,7 +76,8 @@ SYM_FUNC_END(vmx_vmenter)
>   */
>  SYM_FUNC_START(vmx_vmexit)
>  #ifdef CONFIG_RETPOLINE
> -	ALTERNATIVE "jmp .Lvmexit_skip_rsb", "", X86_FEATURE_RETPOLINE
> +	ALTERNATIVE "RET", "", X86_FEATURE_RETPOLINE
> +
>  	/* Preserve guest's RAX, it's used to stuff the RSB. */
>  	push %_ASM_AX
>  
> @@ -87,7 +88,6 @@ SYM_FUNC_START(vmx_vmexit)
>  	or $1, %_ASM_AX
>  
>  	pop %_ASM_AX
> -.Lvmexit_skip_rsb:
>  #endif
>  	RET
>  SYM_FUNC_END(vmx_vmexit)
> -- 
> 2.37.0.rc0.161.g10f37bed90-goog
>