On Thu, Jan 11, 2018 at 12:04:35AM +0000, Woodhouse, David wrote:
> On Wed, 2018-01-10 at 15:47 -0800, Tim Chen wrote:
> >
> > > +
> > > + asm volatile (ALTERNATIVE("",
> > > + __stringify(__FILL_RETURN_BUFFER(%0, %1,
> > > _%=)),
> > > + X86_FEATURE_RETPOLINE)
> >
> > We'll be patching in a fairly long set of instructions here. Maybe put
> > the ALTERNATIVE in the assembly and use a jmp skip_\@ for the ALTERNATIVE.
>
> Perhaps the alternatives.h header could give me a clean way of doing this:
>
> --- a/arch/x86/include/asm/nospec-branch.h
> +++ b/arch/x86/include/asm/nospec-branch.h
> @@ -177,7 +178,7 @@ static inline void vmexit_fill_RSB(void)
> {
> unsigned long dummy;
>
> - 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" );
Or we teach the alternative thing to patch in a jmp to end instead of
NOP padding the entire thing as soon as the jmp (3 bytes) fits ?
