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

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Date:	Thu, 22 May 2008 14:02:28 -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:
> 
> hpa, What is the virtualization problem? Are you referring to perf problem?
> As you noted, regular non-rt signal handlers won't need this cpuid check. It's
> needed only for those who manually look at non-rt signal frames and interpret it.
> And also, they can do this check only once and not everytime.
> 

No, relying on CPUID and vdso both have implications for virtualization.

> To me, prtcl() just seems to be an overkill.

I don't think it is ... it's not overkill but rather "underkill"... it's 
a low-performance solution but it's guaranteed to be safe in the 
presence of virtualization of all its various ilk.  Note that you don't 
need to be able to *set* the format via prctl(), just *query* (get) it.

Using prctl() allows us to make this personality-dependent if we ever 
need to.

> While restoring from the user, kernel also need to find out what layout
> the user is passing. So it's bi-directional. I prefer the same mechanism
> (using cookies/magic numbers etc inaddition to uc_flags or cpuid checks) to
> interpret the fpstate for both user/kernel.

No, it really doesn't: the kernel only needs to be able to read the same 
format as it itself wrote.

> ARM also seem to be using similar things while extending their ucontext_t,
> with out other kernel interfaces to indicate the layout.
> 
> BTW, how come 32bit kernel doesn't have the X86_FXSR_MAGIC checks, while restoring
> the extended fxsave data from _fpstate?

Again, the kernel already knows the format, so it doesn't need to check.

	-hpa
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