Re: GDB with PCIe device

[Rajinikanth Pandurangan <rajinikanth.p@gmail.com>]

Thanks Simon and Aktemur for the details and pointers.

What is the role of gdbserver here?  I thought its needed only when gdb
client and target machines are connected via serial/ethernet.  Do we need
gdbserver when we debug GPU kernels that are just over pcie?

Thanks in advance!

On Mon, Jan 11, 2021 at 1:31 AM Aktemur, Tankut Baris <
tankut.baris.aktemur@intel.com> wrote:

> On Friday, January 8, 2021 4:18 PM, Simon Marchi wrote:
> > On 2020-12-26 1:48 a.m., Rajinikanth Pandurangan via Gdb wrote:
> > > Hello,
> > >
> > > As per my understanding, gdb calls ptrace system calls which intern
> uses
> > > kernel implementation of architecture specific action (updating debug
> > > registers,reading context memory...) to set breakpoints, and so on.
> > >
> > > But in case of running gdb with PCIe devices such as gpu or fpga, how
> does
> > > the hardware specific actions are being done?
> > >
> > > Should device drivers provide ptrace equivalent kernel implementation?
> > >
> > >  Could any of the gdb gurus shed some light on debug software stacks in
> > > debugging software that runs on one of the mentioned pcie devices?
> > >
> > > Thanks in advance,
> > >
> >
> > One such gdb port that is in development is ROCm-GDB, by AMD:
> >
> >   https://github.com/ROCm-Developer-Tools/ROCgdb
> >
> > It uses a helper library to debug the GPU threads:
> >
> >   https://github.com/ROCm-Developer-Tools/ROCdbgapi
> >
> > I don't want to get too much into how this library works, because I'm
> > sure I'll say something wrong / misleading.  You can look at the code.
> > But I'm pretty sure the GPU isn't debugged through ptrace.
> > The library communicates with the kernel driver somehow, however.
> >
> > So, the GPU devices can use whatever debug interface, as long as a
> > corresponding target exist in GDB to communicate with it.
> >
> > Today, one GDB can communicate with multiple debugging target, but only
> > with one target per inferior.  So you can be debugging a local program
> > while debugging another remote program.
>
> We (Intel) use this approach.  The host program that runs on the CPU is
> represented as an inferior with the native target, and the kernel that
> runs on the GPU is represented as another inferior with a remote target.
> The remote target is connected to an instance of gdbserver that uses a
> GPU-specific debug interface, which is not ptrace.
>
> A high-level presentation is available at
> https://dl.acm.org/doi/abs/10.1145/3388333.3388646
> in case you want more information.
>
> Regards
> -Baris
>
> >
> > In the GPU / coprocessor programming world, the model is often that you
> > run a program on the host, which spawns some threads on the GPU /
> > coprocessor.  From the point of view of the user, the threads on the host
> > and the threads on the GPU / coprocessor belong to the same program, so
> > would ideally appear in the same inferior.  ROCm-GDB does this, but it's
> > still done in a slightly hackish way, where the target that talks to the
> > GPU is installed in the "arch" stratum (this is GDB internal stuff) of
> > the inferior's target stack and hijacks the calls to the native Linux
> > target.
> >
> > The better long term / general solution is probably to make GDB able to
> > connect to multiple debug targets for a single inferior.
> >
> > Simon
>
>
>
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