linux-kernel - Re: [PATCH 1/2] x86_64,entry: Filter RFLAGS.NT on entry from userspace

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Date:	Tue, 30 Sep 2014 15:42:38 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Thomas Gleixner <tglx@...utronix.de>,
	Andy Lutomirski <luto@...capital.net>
CC:	Sebastian Lackner <sebastian@...-team.de>, X86 ML <x86@...nel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Anish Bhatt <anish@...lsio.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Chuck Ebbert <cebbert.lkml@...il.com>, stable@...nel.org
Subject: Re: [PATCH 1/2] x86_64,entry: Filter RFLAGS.NT on entry from userspace

Nicely enough we get the context switch benefit without any cost in the comon case of 64-bit apps.

On September 30, 2014 3:27:41 PM PDT, Thomas Gleixner <tglx@...utronix.de> wrote:
>On Tue, 30 Sep 2014, Andy Lutomirski wrote:
>> It would certainly be possible to clear NT and retry IRET if IRET
>> fails with NT set.  This would have no overhead for anything
>relevant.
>> That would be this alternative from my 0/2 email:
>> 
>>  - Don't filter NT on sysenter.  Instead, filter it on EFI entry
>>    and modify the IRET code to retry without NT set if NT was set.
>> 
>> Thomas hpa, etc: any thoughts?
>
>Filter it right away. That's solid and obvious. Anything else is just
>complex and prone for future brown paperbag failures.
>
>We get the context switch benefit from it, so there is some
>compensation for the extra cycles.
>
>Thanks,
>
>	tglx

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