linux-kernel - Re: [RFC] x86: xsave/xrstor support, ucontext

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Date:	Wed, 21 May 2008 17:28:41 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Suresh Siddha <suresh.b.siddha@...el.com>
CC:	Mikael Pettersson <mikpe@...uu.se>,
	Andi Kleen <andi@...stfloor.org>, mingo@...e.hu,
	tglx@...utronix.de, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org, roland@...hat.com, drepper@...hat.com,
	Hongjiu.lu@...el.com, linux-kernel@...r.kernel.org,
	arjan@...ux.intel.com, rmk+lkml@....linux.org.uk, dan@...ian.org,
	asit.k.mallick@...el.com
Subject: Re: [RFC] x86: xsave/xrstor support, ucontext_t extensions

Suresh Siddha wrote:
>>
>> An ugly workaround could be to start clearing one of these fields,
>> and say that the data there is only valid for kernels >= 2.6.26.
>> (I said it was ugly...)
>>
>> Or we go back to stashing a flag in uc_flags (which is kosher),
>> and try to figure out how to mark non-rt sigframes.
> 
> This issue of not-zeroing, is present in only 64bit kernels and for 64bit apps,
> right?
> 
> 64bit app signal handling uses only rt_frame, so we can add an uc_flag for
> them and for 32bit apps, kernel was always zero'ing the reserved bits
> at the end of _fpstate.
> 
> In short, for non-rt frames, they can check the reserved bits at the end
> of fpstate frame and for rt-frames (perhaps even for 32bit rt frame handling)
> apps can check for uc_flag aswell, for extended state presence. Is this
> good enough?
> 

Okay, trying to close on this :)

I would suggest using the uc_flag for the rt frames, and simply rely on 
the OSXSAVE flag for non-rt signal frames.  It a rather sucky approach 
(as previously discussed), but since any sane user of these fields (as 
opposed to just relying on the kernel to save/restore) should use the 
SIGINFO frames, I don't see a problem *as long as it's possible to get 
the information* -- any solution which demands performance should just 
turn on SIGINFO and be happy.

The biggest potential problem with this that I see is that relying on 
CPUID can mess with certain virtualization solutions.  Another option to 
accomplish the same thing would be to have a system call (preferrably a 
prctl, since it is at least in theory personality-dependent) to query 
what information is included in the fpstate data - since it will always 
be the same for any particular kernel.

Thoughts?

	-hpa

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