Re: [PATCH RFCv1 08/14] iommufd: Add IOMMU_VIOMMU_SET_DEV_ID ioctl

[Nicolin Chen <nicolinc@nvidia.com>]

On Tue, Jun 11, 2024 at 09:17:56AM -0300, Jason Gunthorpe wrote:
> On Mon, Jun 10, 2024 at 05:44:16PM -0700, Nicolin Chen wrote:
> > On Mon, Jun 10, 2024 at 09:28:39PM -0300, Jason Gunthorpe wrote:
> > > On Mon, Jun 10, 2024 at 04:04:30PM -0700, Nicolin Chen wrote:
> > > 
> > > > > > Actually, even now as we put a dispatcher in VMM, VMM still does
> > > > > > decode the CD table to link ASID to s1_hwpt. Otherwise, it could
> > > > > > only broadcast a TLBI cmd to all pSMMUs.
> > > > > 
> > > > > No, there should be no CD table decoding and no linking ASID to
> > > > > anything by the VMM.
> > > > > 
> > > > > The ARM architecture is clean, the ASID can remain private to the VM,
> > > > > there is no reason for the VMM to understand it.
> > > > 
> > > > But a guest-level TLBI command usually has only ASID available to
> > > > know which pSMMU to dispatch the command. Without an ASID lookup
> > > > table, how could VMM then dispatch a command to the corresponding
> > > > pSMMU?
> > > 
> > > It can broadcast. The ARM architecture does not expect a N:1 mapping
> > > of SMMUs. This is why I think it is not such a good idea..
> > 
> > Hmm, I thought we had an agreed idea that we shouldn't broadcast
> > a TLBI (except global NH_ALL/VAA) for invalidation performance?
> 
> I wouldn't say agree, there are just lots of different trade offs to
> be made here. You can reduce broadcast by parsing the CD table from
> the VMM. You can reduce broadcast with multiple vSMMUs.
> 
> VMM needs to pick a design. I favour multiple vSMMUs.

Yea, having multiple vSMMUs for nesting too seems to be a cleaner
design. The thing is that we have to put a certain complexity in
the VMM, and it should be more efficient by having it at the boot
stage (creating multi-vSMMUs/PCIs and IORT nodes) v.s. runtime
(trappings and distributing at every command).

> > CD table walkthrough would be always done only by VMM, while the
> > lookup table could be created/maintained by the kernel. I feel a
> > vasid table could make sense since we maintain the vdev_id table
> > in the kernel space too.
> 
> I'm not convinced we should put such a micro optimization in the
> kernel. If the VMM cares about performance then split the vSMMU,
> otherwise lets just put all the mess in the VMM and give it the tools
> to manage the invalidation distribution.
> 
> In the long run things like vCMDQ are going to force the choice to
> multi-smmu, which is why I don't see too much value with investing in
> optimizing the single vSMMU case. The optimization can be done later
> if someone does have a use case.

I see. That makes sense to me.

Nicolin