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. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > >