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SubjectRe: [PATCH v2.1] x86/retpoline: Fill return stack buffer on vmexit
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On Thu, 2018-01-11 at 15:32 +0100, Peter Zijlstra wrote:
> On Thu, Jan 11, 2018 at 02:28:32PM +0000, David Woodhouse wrote:
> >
> > On Thu, 2018-01-11 at 08:20 -0600, Josh Poimboeuf wrote:
> > >
> > >
> > > This seems weird.  I liked v1 a lot better.  What's the problem with
> > > patching in the whole thing?
> > >
> > > Also, if you go back to v1, it should be an easy objtool fix, just add
> > > ANNOTATE_NOSPEC_ALTERNATIVE in front of it.
> > The objection was that I was patching in a fairly long set of
> > instructions. I confess I don't actually know why that's a problem,
>
> You get a giant string of NOPs, a single jmp should be faster.

How about this one then (with ANNOTATE_NOSPEC_ALTERNATIVE):

-       asm volatile (ALTERNATIVE("",
+       asm volatile (ALTERNATIVE("jmp " alt_end_marker "f",
                                  __stringify(__FILL_RETURN_BUFFER(%0, %1, _%=)),
                                  X86_FEATURE_RETPOLINE)
                      : "=r" (dummy), ASM_CALL_CONSTRAINT : : "memory" );[unhandled content-type:application/x-pkcs7-signature]
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