Re: [PATCH 3/6] x86: Add support for the clwb instruction

Ross Zwisler
2014-11-11T19:55:10Z

On Tue, 2014-11-11 at 20:46 +0100, Borislav Petkov wrote:
> On Tue, Nov 11, 2014 at 12:40:00PM -0700, Ross Zwisler wrote:
> > Yep, it's weird, I know. :)
>
> But sure, saving opcode space, makes sense to me.
>
> Btw, I'd still be interested about this:
>
> > +static inline void clwb(volatile void *__p)
> > +{
> > + alternative_io_2(".byte " __stringify(NOP_DS_PREFIX) "; clflush %P0",
>
> Any particular reason for using 0x3e as a prefix to have the insns be
> the same size or is it simply because CLFLUSH can stomach it?

Ah, sorry, I was still responding to your first mail. :) Response
copied here to save searching:

Essentially we need one additional byte at the beginning of the
clflush so that we can flip it into a clflushopt by changing that byte
into a 0x66 prefix. Two options are to either insert a 1 byte
ASM_NOP1, or to add a 1 byte NOP_DS_PREFIX. Both have no functional
effect with the plain clflush, but I've been told that executing a
clflush + prefix should be faster than executing a clflush + NOP.

I agree, this is useful info - I'll add it to the patch comments for v2.

Thank you for the feedback.

- Ross