Re: [PATCH 2/2] gdb, gdbserver: Add Intel APX register support

[Keith Seitz <keiths@redhat.com>]

On 7/7/26 10:15 AM, Schimpe, Christina wrote:
>> -----Original Message-----
>> From: Keith Seitz <keiths@redhat.com>
>>
>> I have a (probably naive) question about 'apx_layout' in the various
>> supported configurations. As patched, this member is set when XCR0 has a bit
>> set for PKRU or AVX512 support. Does this mean that EGPR reads from core
>> files with only "plain" AVX support will be incorrect (at offest 0 instead of
>> 960)? AFAICT, the set of features described by XCR0 are independent of each
>> other, e.g., having PKRU does not mean that APX is enabled/supported.
> 
> Yes, in theory every xsave feature is totally independent from each other.
> 
> However, our assumption for known Intel CPUs is that apx is possible
> - with amx (this case is only implemented for gdb-oneapi, since amx is not yet merged upstream)
> - with pkru (without amx)
> - with avx512 (without pkru, amx)
> 
>> This may be a paper-only issue, but perhaps a little defensive programming
>> might be in order?
> 
> This function should be replaced one day with a proper solution.
> 
> See
> "The long term solution is to pull the CPUID info from gdbserver in the form of NT_X86_CPUID, but in the short term we could just always use the Intel layout that GDB assumed before the XSAVE series.  That is, we could pick a default layout based on the XCR0 mask that corresponds to one of the known sizes and use that to set both the layout and size.  I'll see if I can't come up with a variant of that today or tomorrow."
> https://sourceware.org/bugzilla/show_bug.cgi?id=30912
> 
> I believe for now we must live with that heuristic. Adding @Simon Marchi to confirm.
> 

Ah, thank you for pointing me to previous context. I did not remember
any of that.

In which case, I don't have further concerns with the patch with
the previous problems addressed.

Thanks!
Keith