Re: PATCH: 6/6 [2nd try]: Add AVX support (gdbserver changes)

Pedro Alves
2010-03-28T07:55:56Z

On Sunday 28 March 2010 02:11:31, H.J. Lu wrote:
> > I guess you haven't tested this one :-) You may want to add an AVX
> > test to the testsuite, if it's not too much trouble. You're checking
> > for the "x86=xml" feature in the target, but only calling the target
> > method for "x86:xstate=...". I don't see how it could work.
> >
> > The problem we're solving by modifying qSupported is that older
> > versions of GDB, which do not support XML registers at all, assume
> > a specific layout for the g/G packet. Newer versions, which do
> > support XML, will use whatever the target supplies. So, you only want
> > the target to supply the registers via XML if GDB will understand
> > them. Is that accurate?
>
> Yes,
>
> > If that's the scope of the problem, then how about we handle
> > this in a way we can reuse for other targets? That doesn't have
> > to change the implementation; just rename the feature to
> > "xmlRegisters+".
>
> I will make the change.

This (and the gdbarch_qsupported mechanism) worries me multi-arch
design wise. There's a bootstrapping problem here. GDB sends qSupported
to the target before knowing the target's target description. The target
sends the target description based on qSupported.
As is, things only work correctly, when GDB already somehow knows the
arch is some sort of x86 _before_ connecting to the target. That's
usually true if you give GDB a binary, but may not be true in some
use cases.

As a matter of example, if you have, say, a PPC --enable-targets=all
GDB build, and you simply do:

$ gdb
(gdb) tar rem :9999

to connect to a x86 linux gdbserver, then, the x86 target will not
be sending the registers target description, because GDB wouldn't
be sending the "x86=xml" feature (the target_gdbarch would be
set to something not-x86 early in the connection, at the point
gdbarch_qsupported it called). With the "xmlRegisters+" change,
it would be slightly even worse, as GDB would be sending a generic
"xmlRegisters+", meaning "Hello target, I understand xml register
descriptions for your arch", but, at a point when it may be
mistaken what is the target's arch, and the target would
have no way of knowing that.

It seems to me that GDB should be sending "x86=xml" or something
similar to the target unconditionally of whatever target_gdbarch is
before having fetched the target description.

What do you guys think? Did I miss this being discussed?

--
Pedro Alves