Messages in this thread | | | Date | Wed, 29 Nov 2023 08:20:53 +0100 | From | Peter Zijlstra <> | Subject | Re: [RFC] x86/kvm/emulate: Avoid RET for fastops |
| |
On Tue, Nov 28, 2023 at 05:37:52PM -0800, Sean Christopherson wrote: > 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).
That's encouraging..
> > 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. :-/
I'll go make that happen. Thanks!
| |