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

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Date:	Tue, 20 May 2008 10:53:25 -0700
From:	Suresh Siddha <suresh.b.siddha@...el.com>
To:	Mikael Pettersson <mikpe@...uu.se>
Cc:	"H. Peter Anvin" <hpa@...or.com>, Andi Kleen <andi@...stfloor.org>,
	Suresh Siddha <suresh.b.siddha@...el.com>, 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

On Tue, May 20, 2008 at 05:20:43PM +0200, Mikael Pettersson wrote:
> H. Peter Anvin writes:
>  > Mikael Pettersson wrote:
>  > > So the question now is what if anything has the Linux kernel written
>  > > to those reserved fields. (Looking..) Hmm, signal delivery on x86-64
>  > > seems to do fxsave directly to the fxsave area in the user's sigframe,
>  > > which would imply that the reserved fields have unpredictable values.
>  > > 
>  > 
>  > OK, so that's not a usable path unless we can find some area in the 
>  > existing data set to put a flag.  Groan.
> 
> 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?

thanks,
suresh
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