linux-kernel - Re: [PATCH 1/1] x86/elf: Add a new .note section containing Xfeatures information to x86 core files

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Message-ID: <dd54d6de-0bcc-4b2e-a420-b1a429b06246@intel.com>
Date: Thu, 14 Mar 2024 08:37:09 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: Vignesh Balasubramanian <vigbalas@....com>, linux-kernel@...r.kernel.org,
 linux-toolchains@...r.kernel.org
Cc: mpe@...erman.id.au, npiggin@...il.com, christophe.leroy@...roup.eu,
 aneesh.kumar@...nel.org, naveen.n.rao@...ux.ibm.com, ebiederm@...ssion.com,
 keescook@...omium.org, x86@...nel.org, linuxppc-dev@...ts.ozlabs.org,
 linux-mm@...ck.org, bpetkov@....com, jinisusan.george@....com, matz@...e.de,
 binutils@...rceware.org, jhb@...eBSD.org, felix.willgerodt@...el.com
Subject: Re: [PATCH 1/1] x86/elf: Add a new .note section containing Xfeatures
 information to x86 core files

On 3/14/24 04:23, Vignesh Balasubramanian wrote:
> Add a new .note section containing type, size, offset and flags of
> every xfeature that is present.

Mechanically, I'd much rather have all of that info in the cover letter
in the actual changelog instead.

I'd also love to see a practical example of what an actual example core
dump looks like on two conflicting systems:

   * Total XSAVE size
   * XCR0 value
   * XSTATE_BV from the core dump
   * XFEATURE offsets for each feature

Do you have any information about what other OSes are doing in this
area?  I thought Windows, for instance, was even less flexible about the
XSAVE format than Linux is.

Why didn't LWP cause this problem?

>From the cover letter:

> But this patch series depends on heuristics based on the total XSAVE
> register set size and the XCR0 mask to infer the layouts of the
> various register blocks for core dumps, and hence, is not a foolproof
> mechanism to determine the layout of the XSAVE area.

It may not be theoretically foolproof.  But I'm struggling to think of a
case where it would matter in practice.  Is there any CPU from any
vendor where this is actually _needed_?

Sure, it's ugly as hell, but these notes aren't going to be available
universally _ever_, so it's not like the crummy heuristic code gets to
go away.

Have you seen the APX spec?

>
https://www.intel.com/content/www/us/en/developer/articles/technical/advanced-performance-extensions-apx.html

It makes this even more fun because it adds a new XSAVE state component,
but reuses the MPX offsets.

> This information will be used by the debuggers to understand the XSAVE
> layout of the machine where the core file is dumped, and to read XSAVE
> registers, especially during cross-platform debugging.

This is pretty close to just a raw dump of the XSAVE CPUID leaves.
Rather than come up with an XSAVE-specific ABI that depends on CPUID
*ANYWAY* (because it dumps the "flags" register aka. ECX), maybe we
should just bite the bullet and dump out (some of) the raw CPUID space.