Re: [PATCH v3 0/8] gdbserver improvements for AArch64 SVE support

Simon Marchi via Gdb-patches
2023-02-06T20:05:45Z

> Ah! I had also suggested to you something like that at the Cauldron (when we were in line
> for lunch. :-D However, IIRC, I had suggested that we should be able to cache the tdesc by filename.
>
> Let me explain -- when we fetch a target description, we actually tell the server
> to retrieve a tdesc _by a given filename_. By default, we ask for target.xml, like this:
>
> qXfer:features:read:target.xml
>
> but then, the retrieved target.xml file may "xi:include" some other file, like for example these do:
>
> gdb/features/s390-linux64.xml:14: <xi:include href="s390-core64.xml"/>
> gdb/features/s390-linux64.xml:15: <xi:include href="s390-acr.xml"/>
> gdb/features/s390-linux64.xml:16: <xi:include href="s390-fpr.xml"/>
>
> (try grepping for xi:include in the gdb/features/ dir for a lot more hits.)
>
> and so when processing each of those xi:include's, gdb sends another qXfer:features:read packet,
> with the corresponding included filename, like e.g.,
>
> qXfer:features:read:s390-core64.xml
>
> Here's what the manual says:
>
> @item qXfer:features:read:@var{annex}:@var{offset},@var{length}
> @anchor{qXfer target description read}
> Access the @dfn{target description}. @xref{Target Descriptions}. The
> annex specifies which XML document to access. The main description is
> always loaded from the @samp{target.xml} annex.
>
> So basically I am suggesting that instead a new ID mechanism, we should be able to
> use the preexisting annex/filename concept as key. That means that the stop reply and
> the thread listing would include a new "tdesc=foo.xml" attribute, instead of this
> ID that then is defined to map to "target-id-%u.xml", which is basically admitting
> that tdesc filenames exist anyhow.

Just for completeness, how do you envision that working for SVE?
GDBserver would make up unique names for each configuration, like
"target-vq-%d.xml"?

If caching using filenames as keys, what is the scope of that namespace?
Per remote connection, per inferior? I think it wouldn't work per
remote connection, because fetching "target.xml" for two different
inferiors could give two different answers.

Tangentially, I'm wondering if querying qXfer:features:read (mostly
fetching "target.xml" is going to become thread-sensitive. In other
words, if GDB set the general thread to a thread with vq == A, gets
target.xml, then sets the general thread to a thread with vq == B, then
gets target.xml, is it going to get two different target descriptions?
I think that it would make sense to do so *. Therefore, caching
target.xml per-inferior wouldn't be reliable either. And if included
files could vary per thread, you'd have to make sure to give them unique
names.

* I think that since we're going towards thread-specific tdescs, the
process-wide tdesc is going to become an obsolete concept, maybe just
kept for backwards compatibility for when an old GDB not aware of
per-thread tdescs talks to a new GDBserver aware of it, or vice versa.

Simon