RE: [PATCH v3 10/44] gdb, gdbserver, rsp, ze: acknowledge libraries

["Metzger, Markus T" <markus.t.metzger@intel.com>]

Hello Simon,

>> From: Markus Metzger <markus.t.metzger@intel.com>
>>
>> On some accelerator devices, device shared libraries are loaded from a
>> host thread rather than from a device thread.  The reporting entity may
>> not be the one that actually does the load.
>>
>> Intel GPU devices, for example, that are based on Level-Zero, will report
>> shared library events via the device's debug interface.  This is triggered
>> from a host thread calling the run-time interface for loading a device
>> shared library.
>>
>> The Level-Zero run-time ensures that this host thread will not return
>> until the respective debug event has been acknowledged by the debugger.
>> This allows debuggers to set breakpoints before the new library is used.
>>
>> Add a mechanism that allows gdbserver to request acknowledgement of
>newly
>> reported shared libraries and GDB to acknowledge requested libraries after
>> placing breakpoints.
>
>I had trouble understanding the need for this until I remembered that
>the remote inferior only has the GPU threads.  So the thread loading the
>library isn't even part of the inferior.
>
>Otherwise, I think it would just naturally work:
>
> - host thread hits magic breakpoint
> - gdbserver fetches event, notices new library loaded, returns "library
>   loaded" event to gdb
> - gdb loads the new solibs, then resumes the host thread
> - gdbserver resumes the host thread, allowing the application to
>   use the newly loaded library

Indeed, that's how it would work if we involved the host process.

In our case, however, the user-mode driver notifies the kernel-mode driver
about modules.  The debugger will receive those as events.

To ensure that the debugger has had a chance to place breakpoints before
the host process submits those kernels, we require the debugger to acknowledge
such a module load event.  The kernel mode driver will keep the host process
thread stopped in the module load ioctl() until the load has been acknowledged.

In your case, the breakpoint inside the host process loader does that.

Regards,
Markus.
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