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

[Dave Hansen]

On 3/14/24 09:08, Borislav Petkov wrote:
> On Thu, Mar 14, 2024 at 08:37:09AM -0700, Dave Hansen wrote:
>> 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.