linux-kernel - Re: [RFC PATCH 5/6] iommu: Support mm PASID 1:1 with sva domain

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Date:   Tue, 11 Jul 2023 11:18:54 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Baolu Lu <baolu.lu@...ux.intel.com>
Cc:     Tina Zhang <tina.zhang@...el.com>,
        Kevin Tian <kevin.tian@...el.com>,
        Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
        Michael Shavit <mshavit@...gle.com>, iommu@...ts.linux.dev,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 5/6] iommu: Support mm PASID 1:1 with sva domain

On Tue, Jul 11, 2023 at 10:43:43AM +0800, Baolu Lu wrote:
> On 2023/7/11 1:28, Jason Gunthorpe wrote:
> > > @@ -88,31 +98,41 @@ struct iommu_sva *iommu_sva_bind_device(struct device *dev, struct mm_struct *mm
> > >   		goto out_unlock;
> > >   	}
> > > -	if (domain) {
> > > -		domain->users++;
> > > -		goto out;
> > > +	if (unlikely(domain)) {
> > > +		/* Re-attach the device to the same domain? */
> > > +		if (domain == sva_domain) {
> > > +			goto out;
> > > +		} else {
> > > +			/* Didn't get detached from the previous domain? */
> > > +			ret = -EBUSY;
> > > +			goto out_unlock;
> > > +		}
> > >   	}
> > And if we do all of this we should just get rid of the horrible
> > iommu_get_domain_for_dev_pasid() entirely.
> 
> At the core level, we have no idea about whether an sva domain allocated
> for one device is compatible with another device. Hence, we should loop
> the sva domains in the list and if the attach interface feeds back
> -EINVAL's (not compatible), we should allocate a new domain for the
> attached device and put it in the list if the new attachment is
> successful.

Yes, generally.

It would be good to undertand if the drivers desire one sva domain per
instance or one sva domain per driver - but with this approach it
becomes a driver choice which to use. I would guess that one sva
domain per instance is slightly simpler in the drivers..

iommu_get_domain_for_dev_pasid() turns into a check if the group
already has a SVA domain bound from a list, ie we turn it upside down
and have it search the list under the xa_lock instead of trying to
return a domain pointer back out.

Jason