Re: [PATCH v3 0/2] x86_64,entry: Clear NT on entry and speed up switch_to

From: "H. Peter Anvin" <hpa@zytor.com>
To: Andy Lutomirski <luto@amacapital.net>,
	Thomas Gleixner <tglx@linutronix.de>, X86 ML <x86@kernel.org>,
	Ingo Molnar <mingo@redhat.com>
Cc: Sebastian Lackner <sebastian@fds-team.de>,
	Anish Bhatt <anish@chelsio.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Chuck Ebbert <cebbert.lkml@gmail.com>
Subject: Re: [PATCH v3 0/2] x86_64,entry: Clear NT on entry and speed up switch_to
Date: Wed, 01 Oct 2014 11:34:10 -0700	[thread overview]
Message-ID: <542C4922.408@zytor.com> (raw)
In-Reply-To: <cover.1412187233.git.luto@amacapital.net>

On 10/01/2014 11:28 AM, Andy Lutomirski wrote:
> Anish Bhatt noticed that user programs can set RFLAGS.NT before
> syscall or sysenter, and the kernel entry code doesn't filter out
> NT.  This causes kernel C code and, depending on thread flags, the
> exit slow path to run with NT set.
> 
> The former is a little bit scary (imagine calling into EFI with NT
> set), and the latter will fail with #GP and send a spurious SIGSEGV.
> 
> One answer would be "don't do that".  But the kernel can do better
> here.
> 
> These patches filter NT on all kernel entries.  For syscall (both
> bitnesses), this is free.  For sysenter, it seems to cost very
> little (less than my ability to measure, although I didn't try that
> hard).  Patch 2, which isn't tagged for -stable, speeds up context
> switches by avoiding saving and restoring flags, so this series
> should be a decent overall performance win.
> 
> See: https://bugs.winehq.org/show_bug.cgi?id=33275
> 
> Note to bikeshedders: I have no desire to go crazy micro-optimizing
> the sysenter path. :) This version seems to be good enough (and
> should be a performance *increase* for most workloads).
> 

The motivation for this in -stable is the Wine issue, right?  Could you
please add that to the patch description for the 1/2 patch?

Thanks,

	-hpa