Re: [PATCH v2 11/14] target/arm: default SVE length to 64 bytes for linux-user

From: Richard Henderson <richard.henderson@linaro.org>
To: "Alex Bennée" <alex.bennee@linaro.org>
Cc: damien.hedde@greensocs.com,
	Peter Maydell <peter.maydell@linaro.org>,
	luis.machado@linaro.org, qemu-devel@nongnu.org,
	"open list:ARM TCG CPUs" <qemu-arm@nongnu.org>,
	alan.hayward@arm.com
Subject: Re: [PATCH v2 11/14] target/arm: default SVE length to 64 bytes for linux-user
Date: Thu, 5 Dec 2019 11:36:26 -0800	[thread overview]
Message-ID: <cc54dbf6-01ed-f92d-5088-efe7e917b870@linaro.org> (raw)
In-Reply-To: <87o8wm7k6v.fsf@linaro.org>

On 12/5/19 9:31 AM, Alex Bennée wrote:
> 
> Richard Henderson <richard.henderson@linaro.org> writes:
> 
>> On 11/30/19 8:45 AM, Alex Bennée wrote:
>>> The Linux kernel chooses the default of 64 bytes for SVE registers on
>>> the basis that it is the largest size that won't grow the signal
>>> frame. When debugging larger sizes are also unwieldy in gdb as each
>>> zreg will take over a page of terminal to display.
>>>
>>> The user can of course always specify a larger size with the
>>> sve-max-vq property on the command line:
>>>
>>>   -cpu max,sve-max-vq=16
>>>
>>> This should not make any difference to SVE enabled software as the SVE
>>> is of course vector length agnostic.
>>>
>>> Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
>>> ---
>>>  target/arm/cpu64.c | 3 +++
>>>  1 file changed, 3 insertions(+)
>>
>> 6 is the largest size that doesn't grow the signal frame.
>> I imagine 4 was chosen because that's the only real hw atm.
>>
>>> +        /* Default sve-max-vq to a reasonable numer */
>>> +        cpu->sve_max_vq = 4;
>>
>> I also agree that we should match the kernel, but this is not the right way.
>> Changing max vq is not the same as changing the default vq.
>>
>> You should change the value of env->vfp.zcr_el[1] in arm_cpu_reset(), and the
>> user can increase the length with prctl(2) as they would be able to on real
>> hardware that would have support for longer vector lengths.
> 
> No the intention is to default to a lower max VQ because...
> 
>> Also, I don't think you should mix this up with gdb stuff.
> 
> it is what we use for sizing the registers for the gdbstub. The other
> option would be to use the effective zcr_el1 value at the time of the
> gdbstub connecting but then things will go horribly wrong if the user
> execute a prctl and widens their size.

Why would you care about the size of the registers as passed by default?  You
shouldn't need or want to change that default to make gdbstub work.

The gdbstub should be passing along the vq value (via the "vg" pseudo-register,
iirc), and gdb should be working out what to display based on that.

If that isn't happening, and you are only changing the default so that gdb
quits displaying massive registers when they aren't in use, then you're doing
something wrong with gdb and gdbstub.

r~