Re: [RFC PATCH 08/10] arm64/sve: ptrace: Wire up vector length control and reporting

[Alan Hayward <Alan dot Hayward at arm dot com>]

> On 17 Jan 2017, at 10:03, Dave Martin <Dave.Martin@arm.com> wrote:
> 
> On Mon, Jan 16, 2017 at 03:11:56PM +0000, Yao Qi wrote:
>> On 17-01-16 13:32:31, Dave Martin wrote:
>>> On Mon, Jan 16, 2017 at 12:20:38PM +0000, Yao Qi wrote:
>>>> On 17-01-12 11:26:07, Dave Martin wrote:
>>>>> This patch adds support for manipulating a task's vector length at
>>>>> runtime via ptrace.
>>>>> 
>>>> 
>>>> I hope kernel doesn't provide such interface to ptracer to change vector
>>>> length.
>>> 
>>> It does, with this patch, beacuse...
>>> 
>>>> The vector length is sort of a read-only property of thread/process/
>>>> program to debugger, unless we really have a clear requirement to modify
>>>> vector length in debugging.  I may miss something because I haven't debug
>>>> SVE code yet.
>>> 
>>> ...the vector length is no longer read-only for the task, thanks to
>>> the new prctls().
>> 
>> What I meant "read-only" is that debugger can't change it, while the program
>> itself can change it via prctl().
> 
> I see.
> 
>>> 
>>> This does add complexity, but I figured that any programmer's model
>>> state that the thread can modify for itself should be modifiable by the
>>> debugger, if for no other reason than the user may want to experiment to
>>> see what happens.  Without a ptrace interface, it would be necessary
>>> to inject a prctl() call into the target, which is possible but awkward.
>> 
>> We only need such interface if it is useful, see more below.
>> 
>> Suppose it is useful to change vector length through ptrace, we should align
>> ptrace interface to prctl() as much as possible.  Looks that both prctl
>> change and ptrace change can go through sve_set_vector_length, easy to keep
>> two consistent.
>> 
>>> 
>>> gdb must already re-detect the vector length on stop, since the target
>>> could have called the prctl() in the meantime.
>> 
>> Yes, gdb assumes the vector length may be changed, so it re-detects on
>> every stop, but I don't see the need for gdb to change the vector length.
>> 
>>> 
>>> Access via ptrace also allows things like trapping on exec, fork or
>>> clone and changing the vector length for the new process or thread
>>> before it starts to run.  I'm guessing here, but such a scenario seems
>>> legitimate (?)
>>> 
>> 
>> Yes, these cases are valid, but the usefulness is still questionable to
>> me.  I just doubt that SVE developers do need to change vector length
>> when they are debugging code.  Note that it is not my strong objection
>> to this patch, if kernel people believe this is useful, I am fine with
>> it.
> 
> That's fair.  I'll leave the patch there for now and see if anyone else
> has a comment to make, but it could be removed without affecting
> anything else.
> 

I would say that whilst it is a very dangerous thing to do and has many
consequences, there is a requirement for a gdb user to be able to change VL
whilst debugging a running process, and I don’t think we should see
changing VL as much different from changing a register value on the fly.

Say you have a loop in assembly you are trying to debug - you might write
to $x2 and then single step to see how this effects the result. With SVE
code you might want to see how different VL values will effect the layout
of results in the vectors, how it effects the predicates and how it changes
the number of iterations the loop makes. Of course, once you exit the
loop all bets are off - just like if you had been changing register values.

The current proposal for gdb is that we will show $VL in the list of
registers, therefore for consistency it’d make sense for the gdb user to
be able to set it as if it was just another register. For this we need a
simple way to change the VL in another process, and I think ptrace() is
the easiest way (given that prctl() only changes its own process).


> Are there situations in which injecting a function call into the target
> won't work, i.e., where we couldn't do:
> 
> set prctl(...)
> 
> ?
> 
> Using the prctl interface this way, it would also be preferable to refer
> to the #defines by name.
> 
> Cheers
> —Dave


Thanks,
Alan.