linux-kernel - Re: [PATCH 02/10] x86/entry/compat: In SYSENTER, sink AC clearing below the existing FLAGS test

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Message-ID: <20160229223722.GI3724@pd.tnic>
Date:	Mon, 29 Feb 2016 23:37:22 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	Brian Gerst <brgerst@...il.com>
Cc:	Andy Lutomirski <luto@...capital.net>,
	Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Andrew Cooper <andrew.cooper3@...rix.com>
Subject: Re: [PATCH 02/10] x86/entry/compat: In SYSENTER, sink AC clearing
 below the existing FLAGS test

On Mon, Feb 29, 2016 at 05:33:16PM -0500, Brian Gerst wrote:
> SYSENTER doesn't save EFLAGS so we have to fudge it by pushing the
> kernel's flags.  Most user programs never set AC, so by adding it to
> the test for flags to clear we can avoid the CLAC or POPF in the
> common case that it is already clear.

Yeah, Andy just explained it to me on IRC. Thanks guys.

-- 
Regards/Gruss,
    Boris.

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