Re: [PATCH] x86_64, asm: Work around AMD SYSRET SS descriptor attribute issue

Borislav Petkov
2015-04-27T18:54:05Z

On Mon, Apr 27, 2015 at 11:47:30AM -0700, Linus Torvalds wrote:
> On Mon, Apr 27, 2015 at 11:38 AM, Borislav Petkov <bp@alien8.de> wrote:
> >
> > So our current NOP-infrastructure does ASM_NOP_MAX NOPs of 8 bytes so
> > without more invasive changes, our longest NOPs are 8 byte long and then
> > we have to repeat.
>
> Btw (and I'm too lazy to check) do we take alignment into account?
>
> Because if you have to split, and use multiple nops, it is *probably*
> a good idea to try to avoid 16-byte boundaries, since that's can be
> the I$ fetch granularity from L1 (although I guess 32B is getting more
> common).

Yeah, on F16h you have 32B fetch but the paths later in the machine
gets narrower, so to speak.

> So the exact split might depend on the alignment of the nop replacement..

Yeah, no. Our add_nops() is trivial:

/* Use this to add nops to a buffer, then text_poke the whole buffer. */
static void __init_or_module add_nops(void *insns, unsigned int len)
{
while (len > 0) {
unsigned int noplen = len;
if (noplen > ASM_NOP_MAX)
noplen = ASM_NOP_MAX;
memcpy(insns, ideal_nops[noplen], noplen);
insns += noplen;
len -= noplen;
}
}

> Can we perhaps get rid of the distinction entirely, and just use one
> set of 64-bit nops for both Intel/AMD?

I *think* hpa would have an opinion here. I'm judging by looking at
comments like this one in the code:

/*
* Due to a decoder implementation quirk, some
* specific Intel CPUs actually perform better with
* the "k8_nops" than with the SDM-recommended NOPs.
*/

which is a fun one in itself. :-)

--
Regards/Gruss,
Boris.

ECO tip #101: Trim your mails when you reply.
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