RE: [PATCH RFC v3 08/12] iommu/sva: Use attach/detach_pasid_dev in SVA interfaces

[Tian, Kevin]

> From: Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx>
> Sent: Wednesday, April 13, 2022 7:58 PM
> On 2022/4/13 7:36, Tian, Kevin wrote:
> >> From: Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx>
> >> Sent: Tuesday, April 12, 2022 8:53 PM
> >>
> >>>
> >>>> +	if (!handle) {
> >>>> +		ret = -ENOMEM;
> >>>> +		goto out_put_ioas;
> >>>> +	}
> >>>> +
> >>>> +	/* The reference to ioas will be kept until domain free. */
> >>>> +	domain = iommu_sva_alloc_domain(dev, ioas);
> >>>
> >>> Shouldn't we first try whether existing domains are compatible to this
> >>> device?
> >>
> >> If we think that here domain represents a hardware pagetable actually
> >> used by IOMMU for a {device, pasid}, we are able to use per-{device,
> >> pasid} domain without checking compatibility. Sharing a domain among
> >> devices under the same IOMMU may be an optimization. That could be
> done
> >> in the IOMMU driver just like what vt-d driver is doing for pass-through
> >> DMA domains.
> >>
> >
> > there is only one hardware page table per mm in this case. Multiple
> domains
> > are required only due to compatibility reason as Jason/Robin pointed out
> > in SMMU case. Given all other places create multiple domains per ioas only
> > upon incompatibility, probably it's more consistent to doing so in this path
> > too...
> 
> Sharing domain for compatible devices is valuable when the domain
> supports map/unmap operations. That can reduce the number of
> map/unmap
> calls and the resulting synchronization of IOTLB. But for SVA case, it's
> a dumb domain which only provides attach/detach operations.
> 
> A similar case could be found on pass-through DMA domains. The iommu
> core allocates a default domain for each group although all the domains
> represent a same page table for the compatible devices. The VT-d driver
> optimizes this by exporting a static identity domain.
> 
> Anyway, I am open for this. I can add a compatible domain list if most
> of you like that way. :-)
> 

This is probably fine as long as such domain is purely dumb. Let's see
whether others have different opinions.