[Mesa-dev] [RFC] Linux Graphics Next: Explicit fences everywhere and no BO fences - initial proposal

Christian König ckoenig.leichtzumerken at gmail.com
Wed Apr 28 13:11:27 UTC 2021

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 at emersion.fr> wrote:
>>>>>> On Tuesday, April 27th, 2021 at 7:31 PM, Lucas Stach <l.stach at pengutronix.de> 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.


>> 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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