[Intel-gfx] [PATCH] drm/i915: Force CPU synchronisation even if userspace requests ASYNC

[Chris Wilson]

Quoting Jason Ekstrand (2017-07-11 01:01:20)
> Given that domain tracking is global, we can also run into interesting issues
> if process A does a CPU map, writes to it, and then hands it to process B which
> uses it from the GPU.  Without being very aggressive about set_domain, we have
> no knowledge that this is a problem.

Yes, set-domain has always been a bit nebulous in that it is a one-sided
barrier for a critical section. If we wanted to start from scratch, it
would instead move the desired region into a cache domain and return an
exclusive fence that the user must then signal. (Exclusive for writing,
shared for concurrent reads.) And if the kernel were to even stop
tracking the cache domain and leave it to userspace that would probably be
even better. (That would have to mean that by convention, the bo is
always coherent when the fence is released, but that could be relaxed
if the only two parties are private.)

Hmm. Actually, this can be inserted into current operations without
breaking backwards compatibility. If we assume old userspace is correctly
calling set-domain prior to accessing the bo through a mmapping (and if
it is not, then it is deliberately using unsynchronised access) then it
will wait for the fences become beginning. The only problem is allowing
userspace to create an unsignaled fence for indefinite periods of time.
The alternative would be something like breaking it after X seconds and
sending a signal to the process (depending on how easy it is to add a
new signal or even if it desirable, that may just be SIGKILL!).

SVM is an interesting thorn in the side. At present, we simply have no
means of implicit tracking (or KMS/prime integration) nor do we have any
means of tracking which pages each operation uses. Hmm, we should have
userspace at least provide a list of bo that are affected by the svm
operation so that the implicit tracking just works. And of course, it
assumes that userspace has made everything coherent. (Or we need to be
even heavier handed and force pagefaults between coherency domains,
that's certainly plausible, see HMM.)
-Chris