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From: Thiago Jung Bauermann <thiago.bauermann@linaro.org>
To: Simon Marchi <simark@simark.ca>
Cc: Pedro Alves <pedro@palves.net>,
	Luis Machado <luis.machado@arm.com>,
	Andrew Burgess <aburgess@redhat.com>,
	Thiago Jung Bauermann via Gdb-patches
	<gdb-patches@sourceware.org>
Subject: Re: [PATCH v3 4/8] gdbserver/linux-aarch64: When thread stops, update its target description
Date: Fri, 10 Feb 2023 03:29:10 +0000	[thread overview]
Message-ID: <87cz6i2o6x.fsf@linaro.org> (raw)
In-Reply-To: <cb67b944-cb83-4316-d005-7a285eeb719a@simark.ca>


Simon Marchi <simark@simark.ca> writes:

>> That is, assuming we continue with the thread-specific tdesc approach
>> rather than the one which expands tdescs to allow describing one
>> register's type in terms of another one. I'll revisit my notes and think
>> more about this.
>
> Can we expand about this idea?  I think I like it, but I don't see 100%
> how it would work.

Sorry, I can't expand much on it. I like it too, and as I mentioned in
another email I spent some time investigating how it could be done but
I wasn't able to make progress so I decided to do the per-thread tdesc
implementation instead.

I would be willing to try again but I would need help with high-level
design.

> I can imagine a vector of registers whose size
> depends directly on the value of some other register that comes before,
> like:
>
>   <vector id="vec" type="some_type" count="some_other_register"/>
>
> Here, "some_other_register" would be a scalar register that comes before
> "vec", and whose value dictates directly the number of elements of
> "vec".  But if you wanted to say that the number of elements in "vec" is
> the value of some_other_register, times 2?  I guess we could write:
>
>   <vector id="vec" type="some_type" count="some_other_register * 2"/>
>
> .. but then we get in the realm of defining a grammar, building a
> parser / evaluator, etc.

We could rein complexity in by supporting only the simplest of
expressions, e.g. only a very rigid form such as “<operand 1> <op>
<operand 2>” where <op> is one of the basic arithmetic operations.
If that turns out to not be enough then we can increasingly support more
complex operations.

> The type of the vector elements needs to be dynamic too, how do
> we define that?

This is the part where I got stuck, especially on how to make GDB's type
system allow expressing such a type.

> If the number of possibilities is known and static, we could have some
> kind of "variant" type, where we list all the possible types, and select
> among them at runtime based on the value of a preceding register.

Yes, in the case of SVE it's known and static. The maximum vector length
is an architectural feature of the processor, and GDB/gdbserver can get
it via ptrace in the NT_ARM_SVE regset. And it's always a multiple of
16.

It's an interesting idea. Perhaps it's enough, at least for SVE?

> If I understand correctly, all of this makes it so the size of the
> response to the g packet will be dynamic too?

We /could/ set the size of the g packet to always correspond to the
largest vector length possible but it would be a big overhead,
especially if there are many threads involved.

So in practice yes, it will be dynamic if we can help it.

-- 
Thiago

  reply	other threads:[~2023-02-10  3:29 UTC|newest]

Thread overview: 94+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2023-01-30  4:45 [PATCH v3 0/8] gdbserver improvements for AArch64 SVE support Thiago Jung Bauermann
2023-01-30  4:45 ` [PATCH v3 1/8] gdbserver: Add assert in find_register_by_number Thiago Jung Bauermann
2023-01-31 17:05   ` Simon Marchi
2023-01-31 19:49     ` Thiago Jung Bauermann
2023-02-01 15:43       ` Simon Marchi
2023-01-30  4:45 ` [PATCH v3 2/8] gdbserver: Add PID parameter to linux_get_auxv and linux_get_hwcap Thiago Jung Bauermann
2023-02-01  9:07   ` Luis Machado
2023-02-01 10:54   ` Andrew Burgess
2023-02-01 16:01     ` Simon Marchi
2023-02-01 19:33       ` Thiago Jung Bauermann
2023-02-01 19:53         ` Simon Marchi
2023-02-01 21:55           ` Thiago Jung Bauermann
2023-02-06 19:54   ` Pedro Alves
2023-02-06 20:16     ` Simon Marchi
2023-02-07 15:19       ` Pedro Alves
2023-02-07 21:47         ` Thiago Jung Bauermann
2023-02-09  1:31           ` Simon Marchi
2023-02-10  3:54             ` Thiago Jung Bauermann
2023-02-07 22:28     ` Thiago Jung Bauermann
2023-01-30  4:45 ` [PATCH v3 3/8] gdbserver/linux-aarch64: Factor out function to get aarch64_features Thiago Jung Bauermann
2023-02-01  8:59   ` Luis Machado
2023-02-01 16:04     ` Simon Marchi
2023-02-01 22:13       ` Thiago Jung Bauermann
2023-01-30  4:45 ` [PATCH v3 4/8] gdbserver/linux-aarch64: When thread stops, update its target description Thiago Jung Bauermann
2023-02-01  9:05   ` Luis Machado
2023-02-01 11:06   ` Andrew Burgess
2023-02-01 16:21     ` Simon Marchi
2023-02-01 16:32       ` Luis Machado
2023-02-02  2:54         ` Thiago Jung Bauermann
2023-02-02  3:47           ` Simon Marchi
2023-02-03  3:47             ` Thiago Jung Bauermann
2023-02-03 11:13               ` Luis Machado
2023-02-04 15:26                 ` Thiago Jung Bauermann
2023-02-03 11:11             ` Luis Machado
2023-02-04 15:21               ` Thiago Jung Bauermann
2023-02-06  9:07                 ` Luis Machado
2023-02-06 12:15                   ` Thiago Jung Bauermann
2023-02-06 20:29                 ` Pedro Alves
2023-02-07  8:11                   ` Luis Machado
2023-02-07 14:39                     ` Thiago Jung Bauermann
2023-02-03 10:57           ` Luis Machado
2023-02-04  6:18             ` Thiago Jung Bauermann
2023-02-06 20:26           ` Pedro Alves
2023-02-07 21:06             ` Thiago Jung Bauermann
2023-02-09  2:46               ` Simon Marchi
2023-02-10  3:29                 ` Thiago Jung Bauermann [this message]
2023-02-10 14:56                   ` Luis Machado
2023-02-10 15:04                     ` Tom Tromey
2023-02-10 15:28                       ` Luis Machado
2023-02-10 17:26                       ` Thiago Jung Bauermann
2023-02-10 21:01                         ` Simon Marchi
2023-01-30  4:45 ` [PATCH v3 5/8] gdbserver: Transmit target description ID in thread list and stop reply Thiago Jung Bauermann
2023-01-30 12:52   ` Eli Zaretskii
2023-01-30 14:05     ` Thiago Jung Bauermann
2023-02-01  9:39   ` Luis Machado
2023-02-01 12:07   ` Andrew Burgess
2023-02-01 13:03     ` Eli Zaretskii
2023-02-01 17:37     ` Simon Marchi
2023-02-02 20:36       ` Thiago Jung Bauermann
2023-02-02 20:56         ` Simon Marchi
2023-02-01 20:46     ` Simon Marchi
2023-02-02 21:43       ` Thiago Jung Bauermann
2023-02-01 14:51   ` Andrew Burgess
2023-02-01 17:03     ` Simon Marchi
2023-02-02 19:52       ` Thiago Jung Bauermann
2023-02-02 20:51         ` Simon Marchi
2023-02-03  2:44           ` Thiago Jung Bauermann
2023-02-03 16:29             ` Andrew Burgess
2023-02-04  6:08               ` Thiago Jung Bauermann
2023-02-03 11:22       ` Luis Machado
2023-02-03 12:50         ` Simon Marchi
2023-01-30  4:45 ` [PATCH v3 6/8] gdb/remote: Parse tdesc field in stop reply and threads list XML Thiago Jung Bauermann
2023-02-01  9:52   ` Luis Machado
2023-02-05  0:06     ` Thiago Jung Bauermann
2023-02-06  9:10       ` Luis Machado
2023-02-01 14:32   ` Andrew Burgess
2023-02-01 19:50     ` Simon Marchi
2023-02-01 20:16       ` Simon Marchi
2023-02-03 11:27         ` Luis Machado
2023-02-03 13:19           ` Simon Marchi
2023-02-03 16:33             ` Andrew Burgess
2023-01-30  4:45 ` [PATCH v3 7/8] gdb/aarch64: Detect vector length changes when debugging remotely Thiago Jung Bauermann
2023-02-01  9:58   ` Luis Machado
2023-02-01 15:26   ` Andrew Burgess
2023-02-01 20:20     ` Simon Marchi
2023-02-03 11:31       ` Luis Machado
2023-02-03 16:38       ` Andrew Burgess
2023-02-03 19:07         ` Simon Marchi
2023-01-30  4:45 ` [PATCH v3 8/8] gdb/testsuite: Add test to exercise multi-threaded AArch64 SVE inferiors Thiago Jung Bauermann
2023-02-01 10:10   ` Luis Machado
2023-02-06 19:11 ` [PATCH v3 0/8] gdbserver improvements for AArch64 SVE support Pedro Alves
2023-02-06 20:05   ` Simon Marchi
2023-02-06 21:06     ` Pedro Alves
2023-02-07 13:49       ` Simon Marchi

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