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

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Date:	Sat, 8 Aug 2015 11:15:08 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Dave Hansen <dave@...1.net>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Anvin <hpa@...or.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org, Andy Lutomirski <luto@...nel.org>,
	bp@...en8.de, Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	fenghua.yu@...el.com, x86@...nel.org, dave.hansen@...ux.intel.com
Subject: Re: [PATCH] x86, fpu: correct XSAVE xstate size calculation


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

> I think we have three options.  Here's some rough pseudo-ish-code to 
> sketch them out.

> /* Option 2: search for the end of the last state, probably works, Ingo likes? */
> 	for (i = 0; i < nr_xstates; i++) {
> 		if (cpu_has_xsaves && !enabled_xstate(i))
> 			continue;
> 		end_of_state = xstate_offsets[i] + xstate_sizes[i];
> 		if (xstate_is_aligned[i]) /* currently not implemented */
> 			end_of_state = ALIGN(end_of_state, 64);
> 		if (end_of_state > total_blob_size)
> 			total_blob_size = end_of_state;
> 	}
> 	/* align unconditionally, maybe??? */
> 	total_blob_size = ALIGN(total_blob_size, 64);
> 
> 	/* Double check our obviously bug-free math with what the CPU says */
> 	if (!cpu_has_xsaves)
> 		cpuid(0xD0, 0, &check_total_blob_size, ...);
> 	else
> 		cpuid(0xD0, 1, &check_total_blob_size, ...);
> 
> 	WARN_ON(check_total_blob_size != total_blob_size);

Yes, this is quite close to what I'd like to see.

So I'd do the following things as well:

 - For each xstate feature we enable, define a data structure on fpu/types.h, and
   double check xstate_size against the sizeof() of that structure. Most of the
   current state components are properly declared, but for example AVX512 is
   missing. This documents the features nicely, and also makes it easy for KGDB 
   and tracers to print out the fields - even if in the normal codepaths we rely 
   on the CPU to handle this structure.

   (This is the most important detail I am and was worried about all along.)

 - If our calculations and that of the CPU's mismatch, pick the CPU's variant
   instead.

 - Use WARN_ONCE() instead of WARN_ON() to make life easier on 100+ core 
   prototypes.

 - Remove the LWP bits, I don't think we'll ever support it, it's a broken
   PMU framework.

 - Detail: at least with AVX-1024 (which I'm sure we'll see one day), the natural
   alignment of vector registers will rise from 64 bytes to 128 bytes. At that 
   point the beginning of the AVX-1024 buffer will likely be two cachelines 
   aligned and there might be more padding at the end of the AVX-512 area.
   Make sure the alignment checking code handles this right.

Thanks,

	Ingo
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