Re: [PATCH v3 0/8] drm: Introduce sparse GEM shmem

[Simona Vetter <simona.vetter@ffwll.ch>]

On Fri, Apr 11, 2025 at 09:52:30AM -0400, Alyssa Rosenzweig wrote:
> > 2. Device Lost
> > --------------
> > 
> > At this point we're left with no other choice than to kill the context.
> > And userspace should be able to cope with VK_DEVICE_LOST (hopefully zink
> > does), but it will probably not cope well with an entire strom of these
> > just to get the first frame out.
> > 
> > Here comes the horrible trick:
> > 
> > We'll keep rendering the entire frame by just smashing one single reserve
> > page (per context) into the pte every time there's a fault. It will result
> > in total garbage, and we probably want to shot the context the moment the
> > VS stages have finished, but it allows us to collect an accurate estimate
> > of how much memory we'd have needed. We need to pass that to the vulkan
> > driver as part of the device lost processing, so that it can keep that as
> > the starting point for the userspace dynamic memory requirement
> > guesstimate as a lower bound. Together with the (scaled to that
> > requirement) gpu driver memory pool and the core mm watermarks, that
> > should allow us to not hit a device lost again hopefully.
> 
> This doesn't work if vertex stages are allowed to have side effects
> (which is required for adult-level APIs and can effectively get hit with
> streamout on panfrost). Once you have anything involving side effects,
> you can't replay work, there's no way to cope with that. No magic Zink
> can do either.

Yeah no attempts at reply, it's just standard gl error handling. So either
tossing the context and reporting that through arb_robustness. Or tossing
the context, "transparently" creating a new one and a mix of recreating
some internal driver objects and thoughts&prayers to give the new context
the best chances possible.

You really want all the tricks in step 1 and the quirks in 3 to make sure
this doesn't ever happen. Or at most once per app, hopefully.

I promised terrible after all :-P

Cheers, Sima
-- 
Simona Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch