linux-kernel - Re: [PATCH] x86, fpu: correct XSAVE xstate size calculation

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Message-ID: <20150805103227.GA3233@gmail.com>
Date:	Wed, 5 Aug 2015 12:32:27 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Dave Hansen <dave@...1.net>
Cc:	dave.hansen@...ux.intel.com, linux-kernel@...r.kernel.org,
	bp@...en8.de, fenghua.yu@...el.com, hpa@...or.com, x86@...nel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andy Lutomirski <luto@...nel.org>,
	Denys Vlasenko <dvlasenk@...hat.com>
Subject: Re: [PATCH] x86, fpu: correct XSAVE xstate size calculation


* Dave Hansen <dave@...1.net> wrote:

> 
> From: Dave Hansen <dave.hansen@...ux.intel.com>
> 
> Note: our xsaves support is currently broken and disabled.  This
> patch does not fix it, but it is an incremental improvement.  It
> might be useful to someone backporting the entire set of XSAVES
> patches at some point, but it should not be backported alone.
> 
> There are currently two xsave buffer formats: standard and
> compacted.  The standard format is waht 'XSAVE' and 'XSAVEOPT'
> produce while 'XSAVES' and 'XSAVEC' produce a compacted-formet
> buffer.  (The kernel never uses XSAVEC)
> 
> But, the XSAVES buffer *ALSO* contains "system state components"
> which are never saved by a plain XSAVE.  So, XSAVES has two
> things that might make its buffer differently-sized from an
> XSAVE-produced one.
> 
> The current code assumes that an XSAVES buffer's size is simply
> the sum of the sizes of the (user) states which are supported.
> This seems to work in most cases, but it is not consistent with
> what the SDM says, and it breaks if we 'align' a component in the
> buffer.  The calculation is also unnecessary work since the CPU
> *tells* us the size of the buffer directly.
> 
> This patch just reads the size of the buffer right out of the
> CPUID leaf instead of trying to derive it.

So how will we know where to find which field, if we cannot even do a size 
calculation?

I realize that the calculation and what CPUID gives us should match, but it's not 
really good for the kernel to not know the precise layout of a critical task 
context data structure ...

So can we turn this into 'double check the CPUID size and print a warning on 
mismatch' kind of boot time sanity check? Preferably for all XSAVE* data formats 
we can run into. I'd be fine with applying such a patch ahead of enabling 
compaction again.

Thanks,

	Ingo
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