On Fri, Aug 11, 2023 at 03:52:52AM +0000, Tian, Kevin wrote: > > From: Nicolin Chen <nicolinc@xxxxxxxxxx> > > Sent: Friday, August 11, 2023 5:03 AM > > > > > > > Is there a use case for invaliation only SW emulated rings, and do we > > > > > care about optimizing for the wrap around case? > > > > > > > > Hmm, why a SW emulated ring? > > > > > > That is what you are building. The VMM catches the write of the > > > producer pointer and the VMM SW bundles it up to call into the kernel. > > > > Still not fully getting it. Do you mean a ring that is prepared > > by the VMM? I think the only case that we need to handle a ring > > is what I did by forwarding the guest CMDQ (a ring) to the host > > directly. Not sure why VMM would need another ring for those > > linearized invalidation commands. Or maybe I misunderstood.. > > > > iiuc the point of a ring-based native format is to maximum code reuse > when later in-kernel fast invalidation path (from kvm to smmu driver) > is enabled. Then both slow (via vmm) and fast (in-kernel) path use > the same logic to handle guest invalidation queue. I see. That's about the fast path topic. Thanks for the input. > But if stepping back a bit supporting an array-based non-native format > could simplify the uAPI design and allows code sharing for array among > vendor drivers. You can still keep the entry as native format then the > only difference with future in-kernel fast path is just on walking an array > vs. walking a ring. And VMM doesn't need to expose non-invalidate > cmds to the kernel and then be skipped. Ah, so we might still design the uAPI to be ring based at this moment, yet don't support a case CONS > 0 to leave that to an upgrade in the future. I will try estimating a bit how complicated to implement the ring, to see if we could just start with that. Otherwise, will just start with an array. Thanks Nic
