linux-kernel - Re: [PATCH RFCv1 08/14] iommufd: Add IOMMU_VIOMMU_SET_DEV

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Date: Mon, 10 Jun 2024 17:44:16 -0700
From: Nicolin Chen <nicolinc@...dia.com>
To: Jason Gunthorpe <jgg@...dia.com>
CC: "Tian, Kevin" <kevin.tian@...el.com>, "will@...nel.org" <will@...nel.org>,
	"robin.murphy@....com" <robin.murphy@....com>,
	"suravee.suthikulpanit@....com" <suravee.suthikulpanit@....com>,
	"joro@...tes.org" <joro@...tes.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "iommu@...ts.linux.dev"
	<iommu@...ts.linux.dev>, "linux-arm-kernel@...ts.infradead.org"
	<linux-arm-kernel@...ts.infradead.org>, "linux-tegra@...r.kernel.org"
	<linux-tegra@...r.kernel.org>, "Liu, Yi L" <yi.l.liu@...el.com>,
	"eric.auger@...hat.com" <eric.auger@...hat.com>, "vasant.hegde@....com"
	<vasant.hegde@....com>, "jon.grimm@....com" <jon.grimm@....com>,
	"santosh.shukla@....com" <santosh.shukla@....com>, "Dhaval.Giani@....com"
	<Dhaval.Giani@....com>, "shameerali.kolothum.thodi@...wei.com"
	<shameerali.kolothum.thodi@...wei.com>
Subject: Re: [PATCH RFCv1 08/14] iommufd: Add IOMMU_VIOMMU_SET_DEV_ID ioctl

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