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

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Message-ID: <CA+55aFzYz=b887TLTuJ1u6Fv3B0eeXtnx+XzojJ8BBqm+Eha_g@mail.gmail.com>
Date:	Mon, 27 Apr 2015 11:47:30 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Borislav Petkov <bp@...en8.de>
Cc:	Andy Lutomirski <luto@...capital.net>,
	Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Denys Vlasenko <vda.linux@...glemail.com>,
	Brian Gerst <brgerst@...il.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Ingo Molnar <mingo@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Will Drewry <wad@...omium.org>,
	Kees Cook <keescook@...omium.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86_64, asm: Work around AMD SYSRET SS descriptor
 attribute issue

On Mon, Apr 27, 2015 at 11:38 AM, Borislav Petkov <bp@...en8.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).

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

> You can recognize the p6_nops being the same as in-the-manual-suggested
> F16h ones.

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

                        Linus
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