[PATCH v1 14/14] iommu/arm-smmu-v3: Add arm_smmu_cache_invalidate_user

[Tian, Kevin]

> From: Nicolin Chen <nicolinc at nvidia.com>
> Sent: Wednesday, March 22, 2023 2:42 PM
> 
> On Tue, Mar 21, 2023 at 08:48:31AM -0300, Jason Gunthorpe wrote:
> > On Tue, Mar 21, 2023 at 08:34:00AM +0000, Tian, Kevin wrote:
> >
> > > > > Rephrasing that to put into a design: the IOCTL would pass a
> > > > > user pointer to the queue, the size of the queue, then a head
> > > > > pointer and a tail pointer? Then the kernel reads out all the
> > > > > commands between the head and the tail and handles all those
> > > > > invalidation commands only?
> > > >
> > > > Yes, that is one possible design
> > >
> > > If we cannot have the short path in the kernel then I'm not sure the
> > > value of using native format and queue in the uAPI. Batching can
> > > be enabled over any format.
> >
> > SMMUv3 will have a hardware short path where the HW itself runs the
> > VM's command queue and does this logic.
> >
> > So I like the symmetry of the SW path being close to that.
> 
> A tricky thing here that I just realized:
> 
> With VCMDQ, the guest will have two CMDQs. One is the vSMMU's
> CMDQ handling all non-TLBI commands like CMD_CFGI_STE via the
> invalidation IOCTL, and the other hardware accelerated VCMDQ
> handling all TLBI commands by the HW. In this setup, we will
> need a VCMDQ kernel driver to dispatch commands into the two
> different queues.
> 

why doesn't hw generate a vm-exit for unsupported CMDs in VCMDQ
and then let them emulated by vSMMU? such events should be rare
once map/unmap are being conducted...