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

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Date:	Wed, 01 Oct 2014 10:53:22 -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>
Subject: Re: [PATCH 1/2] x86_64,entry: Filter RFLAGS.NT on entry from userspace

On 10/01/2014 10:50 AM, H. Peter Anvin wrote:
> On 09/30/2014 04:21 PM, Thomas Gleixner wrote:
>>>
>>> If we ever want those cycles back, I bet that the compat sysenter path
>>> could be trimmed down a lot.  For example, I think that all of the
>>> zero-extension stuff is unnecessary now that we have the magic syscall
>>> wrappers for all (?) syscalls.
>>
>> Emphasis on "(?)". So yes, once we verified that ....
>>
> 
> I don't think that's true.  Many system calls use exactly the same entry
> point for compat and noncompat calls.  I don't see any value in
> replicating that code in every system call.
> 
> The only time we need to do anything horribly special is when we have an
> argument which is a signed long, which fortunately is not at all common.
> 

Also, we still need to shuffle registers around to match the x86-64
calling convention.  Zero extension as part of that shuffle is free.

	-hpa


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