Re: [PATCH 2/2] x86/speculation: Simplify indirect_branch_prediction_barrier()

[David Woodhouse]

On Fri, 2018-01-26 at 21:08 +0100, Borislav Petkov wrote:
> Make it all a function which does the WRMSR instead of having a hairy
> inline asm.

...

> +	alternative_input("",
> +			Â"call __ibp_barrier",
> +			ÂX86_FEATURE_IBPB,
> +			ÂASM_NO_INPUT_CLOBBER("eax", "ecx", "edx", "memory"));
> Â}

Dammit. I know the best time to comment is *before* I add my own sign-
off to it and before Linus has merged it but... I think this is broken.

If you're calling a C function then you have to mark *all* the call-
clobbered registers as, well, clobbered.

If you really really really want to *call* something out of line, then
it would need to be implemented in asm.

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