Am 28.04.21 um 14:26 schrieb Daniel Vetter:
On Wed, Apr 28, 2021 at 02:21:54PM +0200, Daniel Vetter wrote:
On Wed, Apr 28, 2021 at 12:31:09PM +0200, Christian König wrote:
Am 28.04.21 um 12:05 schrieb Daniel Vetter:
On Tue, Apr 27, 2021 at 02:01:20PM -0400, Alex Deucher wrote:
On Tue, Apr 27, 2021 at 1:35 PM Simon Ser <contact@xxxxxxxxxxx> wrote:
On Tuesday, April 27th, 2021 at 7:31 PM, Lucas Stach <l.stach@xxxxxxxxxxxxxx> wrote:
Ok. So that would only make the following use cases broken for now:
- amd render -> external gpu
- amd video encode -> network device
FWIW, "only" breaking amd render -> external gpu will make us pretty
unhappy
I concur. I have quite a few users with a multi-GPU setup involving
AMD hardware.
Note, if this brokenness can't be avoided, I'd prefer a to get a clear
error, and not bad results on screen because nothing is synchronized
anymore.
It's an upcoming requirement for windows[1], so you are likely to
start seeing this across all GPU vendors that support windows. I
think the timing depends on how quickly the legacy hardware support
sticks around for each vendor.
Yeah but hw scheduling doesn't mean the hw has to be constructed to not
support isolating the ringbuffer at all.
E.g. even if the hw loses the bit to put the ringbuffer outside of the
userspace gpu vm, if you have pagetables I'm seriously hoping you have r/o
pte flags. Otherwise the entire "share address space with cpu side,
seamlessly" thing is out of the window.
And with that r/o bit on the ringbuffer you can once more force submit
through kernel space, and all the legacy dma_fence based stuff keeps
working. And we don't have to invent some horrendous userspace fence based
implicit sync mechanism in the kernel, but can instead do this transition
properly with drm_syncobj timeline explicit sync and protocol reving.
At least I think you'd have to work extra hard to create a gpu which
cannot possibly be intercepted by the kernel, even when it's designed to
support userspace direct submit only.
Or are your hw engineers more creative here and we're screwed?
The upcomming hardware generation will have this hardware scheduler as a
must have, but there are certain ways we can still stick to the old
approach:
1. The new hardware scheduler currently still supports kernel queues which
essentially is the same as the old hardware ring buffer.
2. Mapping the top level ring buffer into the VM at least partially solves
the problem. This way you can't manipulate the ring buffer content, but the
location for the fence must still be writeable.
Yeah allowing userspace to lie about completion fences in this model is
ok. Though I haven't thought through full consequences of that, but I
think it's not any worse than userspace lying about which buffers/address
it uses in the current model - we rely on hw vm ptes to catch that stuff.
Also it might be good to switch to a non-recoverable ctx model for these.
That's already what we do in i915 (opt-in, but all current umd use that
mode). So any hang/watchdog just kills the entire ctx and you don't have
to worry about userspace doing something funny with it's ringbuffer.
Simplifies everything.
Also ofc userspace fencing still disallowed, but since userspace would
queu up all writes to its ringbuffer through the drm/scheduler, we'd
handle dependencies through that still. Not great, but workable.
Thinking about this, not even mapping the ringbuffer r/o is required, it's
just that we must queue things throug the kernel to resolve dependencies
and everything without breaking dma_fence. If userspace lies, tdr will
shoot it and the kernel stops running that context entirely.
Thinking more about that approach I don't think that it will work correctly.
See we not only need to write the fence as signal that an IB is
submitted, but also adjust a bunch of privileged hardware registers.
When userspace could do that from its IBs as well then there is nothing
blocking it from reprogramming the page table base address for example.
We could do those writes with the CPU as well, but that would be a huge
performance drop because of the additional latency.
Christian.
So I think even if we have hw with 100% userspace submit model only we
should be still fine. It's ofc silly, because instead of using userspace
fences and gpu semaphores the hw scheduler understands we still take the
detour through drm/scheduler, but at least it's not a break-the-world
event.
Also no page fault support, userptr invalidates still stall until
end-of-batch instead of just preempting it, and all that too. But I mean
there needs to be some motivation to fix this and roll out explicit sync
:-)
-Daniel
Or do I miss something here?
For now and the next hardware we are save to support the old submission
model, but the functionality of kernel queues will sooner or later go away
if it is only for Linux.
So we need to work on something which works in the long term and get us away
from this implicit sync.
Yeah I think we have pretty clear consensus on that goal, just no one yet
volunteered to get going with the winsys/wayland work to plumb drm_syncobj
through, and the kernel/mesa work to make that optionally a userspace
fence underneath. And it's for a sure a lot of work.
-Daniel
--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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