Re: [RFC] x86/kvm/emulate: Avoid RET for fastops

[Sean Christopherson <seanjc@google.com>]

On Sun, Nov 12, 2023, Peter Zijlstra wrote:
> Hi,
> 
> Inspired by the likes of ba5ca5e5e6a1 ("x86/retpoline: Don't clobber
> RFLAGS during srso_safe_ret()") I had it on my TODO to look at this,
> because the call-depth-tracking rethunk definitely also clobbers flags
> and that's a ton harder to fix.
> 
> Looking at this recently I noticed that there's really only one callsite
> (twice, the testcc thing is basically separate from the rest of the
> fastop stuff) and thus CALL+RET is totally silly, we can JMP+JMP.
> 
> The below implements this, and aside from objtool going apeshit (it
> fails to recognise the fastop JMP_NOSPEC as a jump-table and instead
> classifies it as a tail-call), it actually builds and the asm looks
> good sensible enough.
> 
> I've not yet figured out how to test this stuff, but does something like
> this look sane to you guys?

Yes?  The idea seems sound, but I haven't thought _that_ hard about whether or not
there's any possible gotchas.   I did a quick test and nothing exploded (and
usually when this code breaks, it breaks spectacularly).

> Given that rethunks are quite fat and slow, this could be sold as a
> performance optimization I suppose.
> 
> ---
> 
> diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
> index f93e9b96927a..2cd3b5a46e7a 100644
> --- a/arch/x86/include/asm/nospec-branch.h
> +++ b/arch/x86/include/asm/nospec-branch.h
> @@ -412,6 +412,17 @@ static inline void call_depth_return_thunk(void) {}
>  	"call *%[thunk_target]\n",				\
>  	X86_FEATURE_RETPOLINE_LFENCE)
>  
> +# define JMP_NOSPEC						\
> +	ALTERNATIVE_2(						\
> +	ANNOTATE_RETPOLINE_SAFE					\
> +	"jmp *%[thunk_target]\n",				\
> +	"jmp __x86_indirect_thunk_%V[thunk_target]\n",		\
> +	X86_FEATURE_RETPOLINE,					\
> +	"lfence;\n"						\
> +	ANNOTATE_RETPOLINE_SAFE					\
> +	"jmp *%[thunk_target]\n",				\
> +	X86_FEATURE_RETPOLINE_LFENCE)

There needs a 32-bit version (eww) and a CONFIG_RETPOLINE=n version. :-/