Re: [PATCH drm-next v5 03/14] drm: manager to keep track of GPUs VA mappings

[Matthew Brost <matthew.brost@intel.com>]

On Thu, Jun 22, 2023 at 05:07:11PM +0200, Danilo Krummrich wrote:
> On 6/22/23 17:04, Danilo Krummrich wrote:
> > On 6/22/23 16:42, Christian König wrote:
> > > Am 22.06.23 um 16:22 schrieb Danilo Krummrich:
> > > > On 6/22/23 15:54, Christian König wrote:
> > > > > Am 20.06.23 um 14:23 schrieb Danilo Krummrich:
> > > > > > Hi Christian,
> > > > > > 
> > > > > > On 6/20/23 08:45, Christian König wrote:
> > > > > > > Hi Danilo,
> > > > > > > 
> > > > > > > sorry for the delayed reply. I've trying to dig
> > > > > > > myself out of a hole at the moment.
> > > > > > 
> > > > > > No worries, thank you for taking a look anyway!
> > > > > > 
> > > > > > > 
> > > > > > > Am 20.06.23 um 02:42 schrieb Danilo Krummrich:
> > > > > > > > [SNIP]
> > > > > > > > diff --git a/include/drm/drm_gem.h b/include/drm/drm_gem.h
> > > > > > > > index bbc721870c13..5ec8148a30ee 100644
> > > > > > > > --- a/include/drm/drm_gem.h
> > > > > > > > +++ b/include/drm/drm_gem.h
> > > > > > > > @@ -36,6 +36,8 @@
> > > > > > > >   #include <linux/kref.h>
> > > > > > > >   #include <linux/dma-resv.h>
> > > > > > > > +#include <linux/list.h>
> > > > > > > > +#include <linux/mutex.h>
> > > > > > > >   #include <drm/drm_vma_manager.h>
> > > > > > > > @@ -379,6 +381,18 @@ struct drm_gem_object {
> > > > > > > >        */
> > > > > > > >       struct dma_resv _resv;
> > > > > > > > +    /**
> > > > > > > > +     * @gpuva:
> > > > > > > > +     *
> > > > > > > > +     * Provides the list of GPU VAs attached to this GEM object.
> > > > > > > > +     *
> > > > > > > > +     * Drivers should lock list accesses with
> > > > > > > > the GEMs &dma_resv lock
> > > > > > > > +     * (&drm_gem_object.resv).
> > > > > > > > +     */
> > > > > > > > +    struct {
> > > > > > > > +        struct list_head list;
> > > > > > > > +    } gpuva;
> > > > > > > > +
> > > > > > > >       /**
> > > > > > > >        * @funcs:
> > > > > > > >        *
> > > > > > > 
> > > > > > > I'm pretty sure that it's not a good idea to attach
> > > > > > > this directly to the GEM object.
> > > > > > 
> > > > > > Why do you think so? IMHO having a common way to connect
> > > > > > mappings to their backing buffers is a good thing, since
> > > > > > every driver needs this connection anyway.
> > > > > > 
> > > > > > E.g. when a BO gets evicted, drivers can just iterate
> > > > > > the list of mappings and, as the circumstances require,
> > > > > > invalidate the corresponding mappings or to unmap all
> > > > > > existing mappings of a given buffer.
> > > > > > 
> > > > > > What would be the advantage to let every driver
> > > > > > implement a driver specific way of keeping this
> > > > > > connection?
> > > > > 
> > > > > Flexibility. For example on amdgpu the mappings of a BO are
> > > > > groups by VM address spaces.
> > > > > 
> > > > > E.g. the BO points to multiple bo_vm structures which in
> > > > > turn have lists of their mappings.
> > > > 
> > > > Isn't this (almost) the same relationship I introduce with the
> > > > GPUVA manager?
> > > > 
> > > > If you would switch over to the GPUVA manager right now, it
> > > > would be that every GEM has a list of it's mappings (the gpuva
> > > > list). The mapping is represented by struct drm_gpuva (of course
> > > > embedded in driver specific structure(s)) which has a pointer to
> > > > the VM address space it is part of, namely the GPUVA manager
> > > > instance. And the GPUVA manager keeps a maple tree of it's
> > > > mappings as well.
> > > > 
> > > > If you still would like to *directly* (indirectly you already
> > > > have that relationship) keep a list of GPUVA managers (VM
> > > > address spaces) per GEM, you could still do that in a driver
> > > > specific way.
> > > > 
> > > > Do I miss something?
> > > 
> > > How do you efficiently find only the mappings of a BO in one VM?
> > 
> > Actually, I think this case should even be more efficient than with a BO
> > having a list of GPUVAs (or mappings):
> 
> *than with a BO having a list of VMs:
> 
> > 
> > Having a list of GPUVAs per GEM, each GPUVA has a pointer to it's VM.
> > Hence, you'd only need to iterate the list of mappings for a given BO
> > and check the mappings VM pointer.
> > 
> > Having a list of VMs per BO, you'd have to iterate the whole VM to find
> > the mappings having a pointer to the given BO, right?
> > 
> > I'd think that a single VM potentially has more mapping entries than a
> > single BO was mapped in multiple VMs.
> > 
> > Another case to consider is the case I originally had in mind choosing
> > this relationship: finding all mappings for a given BO, which I guess
> > all drivers need to do in order to invalidate mappings on BO eviction.
> > 
> > Having a list of VMs per BO, wouldn't you need to iterate all of the VMs
> > entirely?
> > 

FWIW I agree with Danilo here on basically all points.

- Having GPUVA list per GEM makes a ton of sense wrt eviction and
  invalidation. Xe had this list prior to GPUVA, after GPUVA it is just
  in a common place.
- From a GPUVA to you can resolve a GEM
- GPUVA can have NULL GEM (userptr or sparse binding in Xe)
- From a VM all GPUVA can be resolved via the maple tree

This seems to do everything any reasonable driver would need.

> > > 
> > > Keep in mind that we have cases where one BO is shared with hundreds
> > > of different VMs as well as potentially the number of mappings can
> > > be >10k.
> > > 

Christian - Can you explain this use case? It seems like an odd one but
regardless the GPUVA design still works we'd just have 10k GPUVA in the
BO's list.

> > > > 
> > > > > 
> > > > > Additional to that there is a state maschine associated with
> > > > > the mappings, e.g. if the page tables are up to date or need
> > > > > to be updated etc....
> > > > > 
> > > > > > Do you see cases where this kind of connection between
> > > > > > mappings and backing buffers wouldn't be good enough? If
> > > > > > so, which cases do you have in mind? Maybe we can cover
> > > > > > them in a common way as well?
> > > > > 
> > > > > Yeah, we have tons of cases like that. But I have no idea
> > > > > how to generalize them.
> > > > 
> > > > They could still remain to be driver specific then, right?
> > > 
> > > Well does the mapping has a back pointer to the BO? And can that be
> > > optional NULL if there is no BO?
> > 
> > Yes to both.
> >

In Xe we have GPUVA with NULL BOs for userptrs and for sparse bindings.
It seems to work rather well.

Matt

> > - Danilo
> > 
> > > 
> > > Regards,
> > > Christian.
> > > 
> > > > 
> > > > > 
> > > > > > 
> > > > > > > 
> > > > > > > As you wrote in the commit message it's highly
> > > > > > > driver specific what to map and where to map it.
> > > > > > 
> > > > > > In the end the common case should be that in a VA space
> > > > > > at least every mapping being backed by a BO is
> > > > > > represented by a struct drm_gpuva.
> > > > > 
> > > > > Oh, no! We also have mappings not backed by a BO at all! For
> > > > > example for partial resident textures or data routing to
> > > > > internal hw etc...
> > > > > 
> > > > > You can't be sure that a mapping is backed by a BO at all.
> > > > 
> > > > I fully agree, that's why I wrote "the common case should be
> > > > that in a VA space at least every mapping *being backed by a BO*
> > > > is represented by a struct drm_gpuva".
> > > > 
> > > > Mappings not being backed by an actual BO would not be linked to
> > > > a GEM of course.
> > > > 
> > > > > 
> > > > > > 
> > > > > > > 
> > > > > > > Instead I suggest to have a separate structure for
> > > > > > > mappings in a VA space which driver can then add to
> > > > > > > their GEM objects or whatever they want to map into
> > > > > > > their VMs.
> > > > > > 
> > > > > > Which kind of separate structure for mappings? Another
> > > > > > one analogous to struct drm_gpuva?
> > > > > 
> > > > > Well similar to what amdgpu uses BO -> one structure for
> > > > > each combination of BO and VM -> mappings inside that VM
> > > > 
> > > > As explained above, I think that's exactly what the GPUVA
> > > > manager does, just in another order:
> > > > 
> > > > BO has list of mappings, mappings have pointer to VM, VM has
> > > > list (or actually a maple tree) of mappings.
> > > > 
> > > > You see any advantages or disadvantages of either order of
> > > > relationships? For me it looks like it doesn't really matter
> > > > which one to pick.
> > > > 
> > > > - Danilo
> > > > 
> > > > > 
> > > > > Regards,
> > > > > Christian.
> > > > > 
> > > > > > 
> > > > > > - Danilo
> > > > > > 
> > > > > > > 
> > > > > > > Regards,
> > > > > > > Christian.
> > > > > > > 
> > > > > > > 
> > > > > > 
> > > > > 
> > > > 
> > > 
>