Re: [Patch] libgomp/gcn: Prepare for reverse-offload callback handling

[Tobias Burnus <tobias@codesourcery.com>]

On 12.10.22 19:09, Andrew Stubbs wrote:

> On 12/10/2022 15:29, Tobias Burnus wrote:
>
> Right, sorry, the buffer is circular, but the counter is linear. It
> simplified reservation that way, but it does mean that there's a limit
> to the number of times the buffer can cycle before the counter
> saturates. (You'd need to stream out gigabytes of data to hit the
> limit though.)
Or in other words, you can have 2^32 = 4,294,967,296 (write chunks +
reverse offloads) per kernel launch.
> ...
>> PS: Currently, device stack variables are private and cannot be
>> accessed from the host; this will change in a separate patch. [...]
> So, the patch, as is, is known to be non-functional? How can you have
> tested it? For the addrs_sizes_kind data to be accessible the
> asm("s8") has to be wrong.

I have tested the non-addrs_sizes_kind part only, which permits to run
reverse-offload functions just fine, but only if they do not use
firstprivate or map. — And I actually also tested with the
addrs_sizes_kind part but that unsurprisingly fails hard when trying to
copy the stack data.

> I think the patch looks good, in principle. The use of the existing
> ring-buffer is the right way to do it, IMO. Can we get the manually
> allocated stacks patch in first and then follow up with these patches
> when they actually work?

I stash this patch as: "OK – but ams still want to have a glance once
__builtin_gcn_kernarg_ptr is in".

I terms of having fewer *.diff files around, I of course would prefer to
just change one line in a follow-up commit instead of keeping a full
patch around, but holding off until __builtin_gcn_kernarg_ptr is ready +
the default has changed to non-private stack variables is also fine.

Tobias

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