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Date: Thu, 23 Apr 2015 17:49:26 +0200
From: Borislav Petkov <bp@alien8.de>
To: Andy Lutomirski <luto@amacapital.net>
Cc: Denys Vlasenko <dvlasenk@redhat.com>, Ingo Molnar <mingo@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Steven Rostedt <rostedt@goodmis.org>, "H. Peter Anvin" <hpa@zytor.com>,
        Oleg Nesterov <oleg@redhat.com>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Alexei Starovoitov <ast@plumgrid.com>, Will Drewry <wad@chromium.org>,
        Kees Cook <keescook@chromium.org>, X86 ML <x86@kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] x86/asm/entry/64: better check for canonical address
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On Thu, Apr 23, 2015 at 08:41:15AM -0700, Andy Lutomirski wrote:
> I was rather vague there.  Let me try again:
> 
> If anyone in the AMD camp really cared, we could add a new bug flag
> X86_BUG_SYSRET_NEEDS_CANONICAL_RCX and set it on Intel chips only, so
> we could use alternatives to patch out the check when running on
> sensible AMD hardware.  This would speed the slow path up by a couple
> of cycles on AMD chips.
> 
> Does that make more sense?  We could call it
> X86_BUG_SYSRET_NEEDS_CANONICAL_RIP if that makes more sense.

Actually "...NEEDS_CANONICAL_RCX" makes more sense as this is what we're
going to patch out eventually, if it makes sense - the RIP canonicalness
test is being done as part of SYSRET, just RCX is not being tested.

Tell you what - how about I perf stat this first by commenting out that
couple of instructions on AMD to see whether it brings anything.

Got an idea for a workload other than a kernel build? :-)

Although a kernel build should do a lot of syscalls too...

Thanks.

-- 
Regards/Gruss,
    Boris.

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