RE: [PATCH 1/1] gdb, gdbserver: Add Intel APX register support

> -----Original Message-----
> From: Keith Seitz <[email protected]>
> Sent: Donnerstag, 13. August 2026 20:14
> To: Joos, Christina <[email protected]>; [email protected]
> Subject: Re: [PATCH 1/1] gdb, gdbserver: Add Intel APX register support
> 
> Hi,
> 
> On 7/25/26 3:53 AM, Christina Joos wrote:
> > From: "H.J. Lu" <[email protected]>
> >
> > Intel Advanced Performance Extensions (APX) doubles the number of
> > general-purpose registers (GPRs) from 16 to 32 (r16-r31).
> 
> Thank you for pursuing this. I'm not going to claim that I completely understand
> the nuiance of modern x86_64 features, but generally, the code seems okay to
> me.
> 
> > Those new 64-bit Extended GPRs (EGPRs) do not change the size of the
> > XSAVE area, as they take up the space left behind by the deprecated
> > MPX registers.
> >
> > In contrast to the legacy GPRS, EGPRs are not enabled by default in
> > 64-bit mode but are xcr0 enabled based on APX_F Intel APX state.
> > Define X86_XSTATE_APX_F to support this new configuration.
> >
> > Add gdb and gdbserver registers support for those new 64-bit EGPRs.
> > Also add byte, word and dword pseudo register support.
> 
> I appreciate the simplified approach in this revision, removing the previously
> proposed maint commands. Generating core files is much better/safe approach.
> 
> One trivial nit (no need to repost):
> I previously asked whether apx_offset should be set independently of
> PKRU/AVX512 in i387_guess_xsave_layout()[1]. You explained that APX is only
> expected with PKRU or AVX512 on known Intel CPUs addresses[2].
> 
> A brief comment documenting that assumption would be helpful for future
> readers. 'Cause, as expected, I forgot all about that discussion already! 😝

I just found a bug in this area:
https://sourceware.org/bugzilla/show_bug.cgi?id=34561

Assuming a fix like the one indicated in comment of the bug will be merged
before this patch I think we can add something like this on top:

~~~
diff --git a/gdb/i387-tdep.c b/gdb/i387-tdep.c
index 51459c31dcc..51180714e08 100644
--- a/gdb/i387-tdep.c
+++ b/gdb/i387-tdep.c
@@ -955,6 +955,8 @@ i387_guess_xsave_layout (uint64_t xcr0, size_t xsave_size,
         Otherwise, we are not able to read xsave registers in OS generated
         corefiles.  */
       layout.avx_offset = 576;
+      /* Intel APX can be available on CPUs with PKRU including AMX or not
+        including AMX support.  */
       layout.apx_offset = 960;
       layout.k_offset = 1088;
       layout.zmm_h_offset = 1152;
@@ -974,6 +976,7 @@ i387_guess_xsave_layout (uint64_t xcr0, size_t xsave_size,
     {
       /* Intel CPUs supporting AVX512.  */
       layout.avx_offset = 576;
+      /* Intel APX can be available on CPUs supporting AVX512.  */
       layout.apx_offset = 960;
       layout.k_offset = 1088;
       layout.zmm_h_offset = 1152;
~~~

Or do you have a different comment in mind?

> Reviewed-By: Keith Seitz <[email protected]>
> 
> Keith
> 
> [1] https://sourceware.org/pipermail/gdb-patches/2026-June/228135.html
> [2] https://sourceware.org/pipermail/gdb-patches/2026-July/228491.html

My original plan was to post this patch once again including the fixes for Eli's
feedback + ones pointed out by me here
https://sourceware.org/pipermail/gdb-patches/2026-August/229759.html
and then wait 1-2 weeks and merge it if there is no more feedback.
Since it's only in the x86 area I think it should be fine for me to self-approve here.
 
However, in the meantime this bug mentioned above popped up.
I think it makes sense to wait until this is resolved.

Christina
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