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Date: Mon, 10 Jun 2024 17:44:16 -0700
From: Nicolin Chen <nicolinc@nvidia.com>
To: Jason Gunthorpe <jgg@nvidia.com>
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Subject: Re: [PATCH RFCv1 08/14] iommufd: Add IOMMU_VIOMMU_SET_DEV_ID ioctl
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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?

> Yes the VMM could walk the CD tables too and build up a bitmap of what
> ASIDs are being used by what pSMMUs, and that would be fine for the
> VMM to do, but I wouldn't necessarily recommend it :)

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.

Anyway, it is still an implementation choice, as Kevin remarked.

Thanks
Nicolin