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

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Message-ID: <20150427100934.GE6774@pd.tnic>
Date:	Mon, 27 Apr 2015 12:09:34 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Denys Vlasenko <dvlasenk@...hat.com>
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>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Brian Gerst <brgerst@...il.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 12:07:14PM +0200, Denys Vlasenko wrote:
> 	/* Only three 0x66 prefixes for NOP for fast decode on all CPUs */
> 	ALTERNATIVE	".byte 0x66,0x66,0x66,0x90 \
> 			.byte 0x66,0x66,0x66,0x90 \
> 			.byte 0x66,0x66,0x66,0x90",
> 			"shl	$(64 - (__VIRTUAL_MASK_SHIFT+1)), %rcx \
> 			sar	$(64 - (__VIRTUAL_MASK_SHIFT+1)), %rcx \
> 			cmpq	%rcx, %r11 \
> 			jne	opportunistic_sysret_failed"
> 			X86_BUG_SYSRET_CANONICAL_RCX
> 
> would be better than letting ALTERNATIVE to generate 13 one-byte NOPs.

We already do everything automagically, see:
arch/x86/kernel/alternative.c: optimize_nops()

:-)

-- 
Regards/Gruss,
    Boris.

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