On Wed, Jun 1, 2022 at 2:40 PM Christian König <christian.koenig@xxxxxxx> wrote: > > Hey guys, > > so today Bas came up with a new requirement regarding the explicit > synchronization to VM updates and a bunch of prototype patches. > > I've been thinking about that stuff for quite some time before, but to > be honest it's one of the most trickiest parts of the driver. > > So my current thinking is that we could potentially handle those > requirements like this: > > 1. We add some new EXPLICIT flag to context (or CS?) and VM IOCTL. This > way we either get the new behavior for the whole CS+VM or the old one, > but never both mixed. Any VM wide flag would be a concern because libdrm_amdgpu merges things into a single drm fd for potentially multiple driver (i.e. radv + amdvlk + radeonsi). I'm also not sure what you'd need a VM flag for? > > 2. When memory is unmapped we keep around the last unmap operation > inside the bo_va. > > 3. When memory is freed we add all the CS fences which could access that > memory + the last unmap operation as BOOKKEEP fences to the BO and as > mandatory sync fence to the VM. > > Memory freed either because of an eviction or because of userspace > closing the handle will be seen as a combination of unmap+free. > > > The result is the following semantic for userspace to avoid implicit > synchronization as much as possible: > > 1. When you allocate and map memory it is mandatory to either wait for > the mapping operation to complete or to add it as dependency for your CS. > If this isn't followed the application will run into CS faults > (that's what we pretty much already implemented). > > 2. When memory is freed you must unmap that memory first and then wait > for this unmap operation to complete before freeing the memory. > If this isn't followed the kernel will add a forcefully wait to the > next CS to block until the unmap is completed. > > 3. All VM operations requested by userspace will still be executed in > order, e.g. we can't run unmap + map in parallel or something like this. > > Is that something you guys can live with? As far as I can see it should > give you the maximum freedom possible, but is still doable. > > Regards, > Christian.
