RE: [PATCH v3 7/8] Windows gdb: Implement AVX register support

["Joos, Christina" <christina.joos@intel.com>]

> -----Original Message-----
> From: Joos, Christina
> Sent: Dienstag, 8. September 2026 17:28
> To: 'Hannes Domani' <ssbssa@yahoo.de>; Tom Tromey <tom@tromey.com>
> Cc: gdb-patches@sourceware.org
> Subject: RE: [PATCH v3 7/8] Windows gdb: Implement AVX register support
> 
> > -----Original Message-----
> > From: Hannes Domani <ssbssa@yahoo.de>
> > Sent: Dienstag, 1. September 2026 20:01
> > To: Tom Tromey <tom@tromey.com>
> > Cc: gdb-patches@sourceware.org
> > Subject: Re: [PATCH v3 7/8] Windows gdb: Implement AVX register
> > support
> >
> >  Am Dienstag, 1. September 2026 um 19:48:28 MESZ hat Tom Tromey
> > <tom@tromey.com> Folgendes geschrieben:
> >
> > > >>>>> "Hannes" == Hannes Domani <ssbssa@yahoo.de> writes:
> > >
> > > Hannes> This adds support for the Intel AVX registers on Windows.
> > > Hannes> It enables accessing registers $ymm0 - $ymm15 where they are
> > available.
> > >
> > > Hannes> After this patch gdb.arch/i386-avx.exp passes on windows.
> > >
> > > I'd like it if Christina would review this.
> > >
> > > Hannes> @@ -1339,7 +1339,7 @@ initialize_loadable ()
> > > Hannes>      {
> > > Hannes>        /* Available XState features masked with implemented
> > > Hannes>features.  */
> > > Hannes>        xstate_features = (GetEnabledXStateFeatures ()
> > > Hannes> -            & X86_XSTATE_SSE_MASK);
> > > Hannes> +            & X86_XSTATE_AVX_MASK);
> > > Hannes>        /* The extended XState functions are only needed if
> > > Hannes>the available
> > > Hannes>      features exceed SSE.  */
> > > Hannes>        if ((xstate_features & ~X86_XSTATE_SSE_MASK) == 0)
> > >
> > > Ok, ignore my earlier message for patch 6.  I see what's going on now.
> > >
> > > I don't have any problem with this patch.  I'm not certain I totally
> > > understand it, but that's ok.
> > >
> > > One thing I did see while doing the background reading is that
> > > supposedly the xstate feature flags are vendor-specific.  So does
> > > some check of the vendor ID need to be done?  I do not know.
> >
> > As I understand it (and I might be wrong about that), there are some
> > vendor- specific XState-bits, but stuff regarding AVX and AVX-512 are
> > the same everywhere.
> 
> The MPX state, for instance, is not implemented by AMD (and has been
> removed from GDB in the meantime, since the feature is deprecated).
> For AVX and AVX-512 I can confirm that they are both implemented for AMD
> and Intel.
> For CET I'd need to double check, but we decided to not add it anyways for
> now.
> So using the masks as you do in the 2 patches should be fine.
> 
> Due to AMD not implementing MPX, there are some differences in the xsave
> offsets for AMD and Intel CPUs, see the dedicated handling here:
> - gdb/i387-tdep.c:i387_guess_xsave_layout: This is relevant for corefiles only.
> - gdb/i386-tdep.c: i387_fallback_xsave_layout
> 
> Both functions are called at some point from i386_gdbarch_init.
> 
> I believe for live targets, since you don't hard-code the offsets in your patch
> series but get them using LocateXStateFeature in the end, it should be safe.
> 
> However, we still call i386_gdbarch_init for windows, too.
> So tdep->xsave_layout should still be available but is not consumed or
> configured.
> This it at least what I observed when doing some basic debugging in windows,
> which I just started today.
> At my current level of understanding, I don't think it makes sense to use it for
> Windows.
> 
> Reading
> "The LocateXStateFeature function must be used to find an individual XState
> feature within an extensible CONTEXT structure. Features are not necessarily
> contiguous in memory and applications should not assume the offset between
> two consecutive features will remain constant in the future."
> https://learn.microsoft.com/en-us/windows/win32/api/winbase/nf-winbase-
> locatexstatefeature confirms that I think.
> 

Hi Hannes, 

I believe that this thread might be the right starting point to discuss the per-register locate_xstate_feature calls in get_context_reg_ptr, since it describes the background and documentation of this topic a bit.

You said
"I was also thinking that these look-ups are not ideal.
As I understand it, the offsets for each feature are not fixed values on windows, but they should be the same for all threads on a system.
In that case, maybe we could calculate these offsets for all registers once, put them in a vector, and use that in get_context_reg_ptr?
I'm also assuming that the register numbers always stay the same.
I'm just not sure at which point the offset calculation should be done.
Any suggestions?"

I believe you are right for the fixed offset for all threads on a system, but this is not clear from the official documentation in windows, or did I miss anything here ? Due to that I am not sure if we can assume that.

On linux, we don't have those per-register function calls to fetch the registers, so we also don't have the per-register find_regno calls on gdbserver side. If we would have one function call for the xsave registers, it would shrink the number of locate_xstate_feature calls, too. But this is probably a bigger refactoring.

So, from my perspective, the safest approach would be either to do the refactoring as described above, or to accept those repeated locate_xstate_feature calls.

What do you think?

Christina
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