On 05/07/2021 15:25, Daniel Vetter wrote:
On Mon, Jul 05, 2021 at 09:34:22AM +0100, Tvrtko Ursulin wrote:
On 02/07/2021 20:22, Daniel Vetter wrote:
On Fri, Jul 02, 2021 at 03:31:08PM +0100, Tvrtko Ursulin wrote:
On 01/07/2021 16:10, Matthew Auld wrote:
The CPU domain should be static for discrete, and on DG1 we don't need
any flushing since everything is already coherent, so really all this
Knowledge of the write combine buffer is assumed to be had by anyone involved?
What about this question? For discrete userspace will assume WC and will
know how to flush WC buffer? Or it is assumed the flush will be hit
implicitly? Will this be documented?
The kernel doesn't pick something at random, it's just fixed. So yeah
userspace needs to flush the WC buffer or anything else.
Right, so does that needs to be documented somewhere or thinking is it
is common knowledge? Probably does to be mentioned in conjunction with
the mmap usage.
does is an object wait, for which we have an ioctl. Longer term the
desired caching should be an immutable creation time property for the
BO, which can be set with something like gem_create_ext.
One other user is iris + userptr, which uses the set_domain to probe all
the pages to check if the GUP succeeds, however keeping the set_domain
around just for that seems rather scuffed. We could equally just submit
a dummy batch, which should hopefully be good enough, otherwise adding a
new creation time flag for userptr might be an option. Although longer
term we will also have vm_bind, which should also be a nice fit for
this, so adding a whole new flag is likely overkill.
Execbuf sounds horrible. But it all reminds me of past work by Chris which is surprisingly hard to find in the archives. Patches like:
commit 7706a433388016983052a27c0fd74a64b1897ae7
Author: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
Date: Wed Nov 8 17:04:07 2017 +0000
drm/i915/userptr: Probe existence of backing struct pages upon creation
Jason Ekstrand requested a more efficient method than userptr+set-domain
to determine if the userptr object was backed by a complete set of pages
upon creation. To be more efficient than simply populating the userptr
using get_user_pages() (as done by the call to set-domain or execbuf),
we can walk the tree of vm_area_struct and check for gaps or vma not
backed by struct page (VM_PFNMAP). The question is how to handle
VM_MIXEDMAP which may be either struct page or pfn backed...
commit 7ca21d3390eec23db99b8131ed18bc036efaba18
Author: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
Date: Wed Nov 8 17:48:22 2017 +0000
drm/i915/userptr: Add a flag to populate the userptr on creation
Acquiring the backing struct pages for the userptr range is not free;
the first client for userptr would insist on frequently creating userptr
objects ahead of time and not use them. For that first client, deferring
the cost of populating the userptr (calling get_user_pages()) to the
actual execbuf was a substantial improvement. However, not all clients
are the same, and most would like to validate that the userptr is valid
and backed by struct pages upon creation, so offer a
I915_USERPTR_POPULATE flag to do just that.
Note that big difference between I915_USERPTR_POPULATE and the deferred
scheme is that POPULATE is guaranteed to be synchronous, the result is
known before the ioctl returns (and the handle exposed). However, due to
system memory pressure, the object may be paged out before use,
requiring them to be paged back in on execbuf (as may always happen).
At least with the first one I think I was skeptical, since probing at
point A makes a weak test versus userptr getting used at point B.
Populate is kind of same really when user controls the backing store. At
least these two arguments I think stand if we are trying to sell these
flags as validation. But if the idea is limited to pure preload, with no
guarantees that it keeps working by time of real use, then I guess it
may be passable.
Well we've thrown this out again because there was no userspace. But if
this is requested by mesa, then the _PROBE flag should be entirely
sufficient.
Why probe and not populate? For me probe is weak and implies to give a
guarantee which cannot really be given. If the pointer is not trusted, there
is no reason to think it cannot go bad between creating the buffer (probe)
and actual use. Populate on the other hand could be described as simply
instantiate the backing store with the same caveat mentioned. No guarantees
about the future validity of the backing store in either case should be
implied.
The pointer can also go bad with populate. The only thing probe guarantees
is that "right now I should be able to call get_user_pages and the only
reasons it could fail is ENOMEM". Which is pretty much the same as we
guarantee when we create a normal object.
Neither does guarantee that by the time you execbuf you won't hit an
ENOMEM. Userptr on top also could make the pointer go invalid if userspace
munmaps or does something else funny.
That's pretty much what I wrote so mostly agreed. Modulo that I think
probe guarantees even less than what you wrote above. Due mmu notifier
invalidation definitely less than with the normal buffer objects
(hypothetically, if there was a populate flag for them). So the only
think which I think makes sense is a populate flag with a disclaimer
explaining how the backing store may not be there any more as soon as
the ioctl successfully returns.
Since I don't want to hold up dg1 pciids on this it'd be nice if we could
just go ahead with the dummy batch, if Ken/Jordan don't object - iris is
the only umd that needs this.
I am not up to speed to understand how to PCI ids come into play here, but
what is the suggestion with the dummy batch - to actually submit something
which ends up executing, waking up the GPU etc? Or be crafty and make it
fail after it acquires backing store? Not sure if we have such a spot that
late so just asking to start with. If the plan is to wake up the GPU that's
quite ugly in my opinion. Especially since patch which adds the flag already
exists so shouldn't really be much a delay to sync userspace and i915 merge.
Just submit a real batch with just MI_BATCHBUFFER_END in it.
Okay, so my opinion stands for this to being quite wasteful and flag
should be preferable.
Regards,
Tvrtko
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