Re: [PATCH] armv7m_nvic: set DHCSR.DEBUGEN when debugger is attached

From: "Alex Bennée" <alex.bennee@linaro.org>
To: Peter Maydell <peter.maydell@linaro.org>
Cc: "Valentin Ghita" <valentinghita@google.com>,
	qemu-arm@nongnu.org, qemu-devel@nongnu.org,
	"Philippe Mathieu-Daudé" <f4bug@amsat.org>
Subject: Re: [PATCH] armv7m_nvic: set DHCSR.DEBUGEN when debugger is attached
Date: Fri, 04 Feb 2022 12:33:46 +0000	[thread overview]
Message-ID: <878ruqst2a.fsf@linaro.org> (raw)
In-Reply-To: <CAFEAcA9y1F5X1BQOwzbJgJKPYvroHFM7ivSUOE-07d3S-GE7DA@mail.gmail.com>

Peter Maydell <peter.maydell@linaro.org> writes:

> On Fri, 4 Feb 2022 at 09:28, Alex Bennée <alex.bennee@linaro.org> wrote:
>> Assuming you are happy for the device to act as though a external
>> debugger is attached regardless of the gdbstub state you could use a CPU
>> property on the command line to enable this behaviour. We have some
>> examples for SVE for the 64 bit CPUs (see object_property_add for
>> sve-max-vq). So something like:
>>
>>   -cpu cortex-m3,dhscr=true
>>
>> You would probably want to model the behaviour of DHSCR.C_HALT as well
>> because that is something the core might do to itself if it detects it
>> is running under debug.
>
> This is sounding pretty hacky to me. I think we should either have
> a proper implementation of all of halting debug (probably opt-in,
> with the default being that the gdbstub is transparent to the guest),

So we could flip it and make it a property of gdbstub with transparency
being the default. Then any architecture that wanted to have this
behaviour could query the stub if enabled.

> or we should just say that no, this isn't something we support,
> and if you want gdb to get control when a particular bit of code
> is executed then you should set a breakpoint there.

It's a fairly niche use case but I don't see why we shouldn't assuming
someone is willing to write the code. However I suspect there is quite a
wide range of potential behaviours to model.

> We don't even implement the guest-visible debug parts of the
> architecture (eg architected single-step) yet, incidentally.

Is this just for Aarch32? Because for Aarch64 as far as I'm aware the
v8.0 debug works fine modulo bugs which I sent a fix for:

  Subject: [RFC PATCH] arm: force flag recalculation when messing with DAIF
  Date: Wed,  2 Feb 2022 12:23:53 +0000
  Message-Id: <20220202122353.457084-1-alex.bennee@linaro.org>

Implementing more of the debug functionality is on the list for
supporting Neoverse. We don't currently have a plan for how to deal with
the slowly growing series of PMU features though.

>
> -- PMM

-- 
Alex Bennée