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

On 3/14/24 09:08, Borislav Petkov wrote:

On Thu, Mar 14, 2024 at 08:37:09AM -0700, Dave Hansen wrote:

quoted

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.

Funny you should say that. This was what they had done originally but if
you dump CPUID and you want to add another component in the future which
is *not* described by CPUID, your scheme breaks.

Are you envisioning an *XSAVE* state component that's not described by
CPUID?

Or some _other_ (non-XSAVE) component in a core dump that isn't
described by CPUID?

So the idea is to have a self-describing buffers layout, independent
from any x86-ism. You can extend this in a straight-forward way then
later.

That argument breaks down a bit on the flags though:

	xc.xfeat_flags = xstate_flags[i];

Because it comes _directly_ from CPUID with zero filtering:

	cpuid_count(XSTATE_CPUID, i, &eax, &ebx, &ecx, &edx);
	...
	xstate_flags[i] = ecx;

So this layout is quite dependent on what's in x86's CPUID.