linux-kernel - Re: [PATCH v2 07/36] x86/entry/64/compat: After SYSENTER, move STI after the NT fixup

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Date:	Mon, 12 Oct 2015 19:48:02 +0200
From:	Denys Vlasenko <dvlasenk@...hat.com>
To:	Andy Lutomirski <luto@...capital.net>
CC:	Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Brian Gerst <brgerst@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH v2 07/36] x86/entry/64/compat: After SYSENTER, move STI
 after the NT fixup

On 10/09/2015 09:48 PM, Andy Lutomirski wrote:
> Anyway, it's still a win, but I'll keep playing.  There may be even
> better ways to do this.

Such as this?
Drop OR, check for cleared IF here:

        testl   $X86_EFLAGS_NT, EFLAGS(%rsp)
        jnz     sysenter_fix_flags
        testl   $X86_EFLAGS_IF, EFLAGS(%rsp)
        jz      sysenter_fix_flags
sysenter_flags_fixed:

and make sysenter_fix_flags code set IF in addition to TF clearing.
This way, we don't even have an unconditional RMW op.

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