RFC: ARM: Support for VFP/NEON registers in coredumps

Ulrich Weigand Ulrich.Weigand at de.ibm.com
Fri Mar 25 13:11:16 UTC 2011

Dave Martin <dave.martin at linaro.org> wrote:
> > So to summarize: the kernel will write additional note sections as if
> > generated via user_regset_view, containing the PTRACE_GETVFPREGS data.
> >
> > Note name: "LINUX"
> Why "LINUX" and not "CORE"?  I don't understand the distinction... are
> the "CORE" notes common to all platforms / all ELF implementations?

Because that's what user_regset_view would do; all notes except the
note name of "LINUX".   I'm not completely sure about the rationale,
but presumably it is indeed because the standard notes are more or less
common across multiple platforms.

> > Note type: t.b.d. [*]
> >
> > [*] Looking at elf.h a logical name/value might be:
> >
> > #define NT_ARM_VFP            0x400             /* ARM VFP/NEON
registers */
> >
> >
> > GDB support along those lines ought to be straightforward.
> It's been suggested that the new note should include a version/flags
> field alongside the ptrace-like register dump, so that if the format
> turns out to be inadequate / broken, it can be extended in a
> compatible way.
> However, nothing else in the coredump or the ptrace interface seems to
> have such versioning implementation.  Ptrace gets extended by adding
> more and more ptrace call types instead.  Adding version fields, while
> sensible, seems inconsistent with the current implementation.
> What's your view on adding a flags field to the VFP state dump?

Again, if we want to use (or mimic) the user_regset_view mechanism,
there is no choice in any of this; the content of the note will be
exactly identical to the content of the ptrace buffer.  Since this
is the way everybody else is using, I think we'd have to have really
good arguments for deviating from it; I'm not sure I see those.

The usual way to deal with changes to the register set is to define
*new* regset structures, which then translates to new ptrace commands
and new core files notes, which are used instead of or in addition to
the old ones ...

Mit freundlichen Gruessen / Best Regards

Ulrich Weigand

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