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

[Sean Christopherson <seanjc@google.com>]

On Wed, Jun 22, 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.

I assume the primary justification is purely to avoid the JMP on modern CPUs?

> Preserve the non-sensical correlation of RSB-stuffing with retpoline.

Either omit the blurb about retpoline, or better yet expand on why it's nonsensical
and speculate a bit on why it got tied to retpoline? 

> Signed-off-by: Jim Mattson <jmattson@google.com>
> ---
>  arch/x86/kvm/vmx/vmenter.S | 10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx/vmenter.S b/arch/x86/kvm/vmx/vmenter.S
> index 435c187927c4..39009a4c86bd 100644
> --- a/arch/x86/kvm/vmx/vmenter.S
> +++ b/arch/x86/kvm/vmx/vmenter.S
> @@ -76,7 +76,12 @@ SYM_FUNC_END(vmx_vmenter)
>   */
>  SYM_FUNC_START(vmx_vmexit)
>  #ifdef CONFIG_RETPOLINE
> -	ALTERNATIVE "jmp .Lvmexit_skip_rsb", "", X86_FEATURE_RETPOLINE
> +	ALTERNATIVE "", "jmp .Lvmexit_stuff_rsb", X86_FEATURE_RETPOLINE
> +#endif
> +.Lvmexit_return:
> +	RET
> +#ifdef CONFIG_RETPOLINE
> +.Lvmexit_stuff_rsb:
>  	/* Preserve guest's RAX, it's used to stuff the RSB. */
>  	push %_ASM_AX

There's a comment in the code here about stuffiing before RET, I think it makes
sense to keep that before the RET, i.e. hoist it out of the actual stuffing
sequence so that it looks like:

#ifdef CONFIG_RETPOLINE
	/* IMPORTANT: Stuff the RSB immediately after VM-Exit, before RET! */
	ALTERNATIVE "", "jmp .Lvmexit_stuff_rsb", X86_FEATURE_RETPOLINE
#endif
.Lvmexit_return:
	RET

Ha!  Better idea.  Rather than have a bunch of nops to eat through before the
!RETPOLINE case gets to the RET, encode the RET as the default.  That allows using
a single #ifdef and avoids both the JMP over the RET as well as the JMP back to the
RET, and saves 4 nops to boot.

SYM_FUNC_START(vmx_vmexit)
#ifdef CONFIG_RETPOLINE
	/* IMPORTANT: Stuff the RSB immediately after VM-Exit, before RET! */
	ALTERNATIVE "RET", "", X86_FEATURE_RETPOLINE

	/* Preserve guest's RAX, it's used to stuff the RSB. */
	push %_ASM_AX

	FILL_RETURN_BUFFER %_ASM_AX, RSB_CLEAR_LOOPS, X86_FEATURE_RETPOLINE

	/* Clear RFLAGS.CF and RFLAGS.ZF to preserve VM-Exit, i.e. !VM-Fail. */
	or $1, %_ASM_AX

	pop %_ASM_AX
#endif
	RET
SYM_FUNC_END(vmx_vmexit)