RE: [PATCH 06/17] iommufd/hw_pagetable: Use domain_alloc_user op for domain allocation

[Liu, Yi L]

> From: Nicolin Chen <nicolinc@xxxxxxxxxx>
> Sent: Friday, February 10, 2023 6:22 AM
> 
> On Thu, Feb 09, 2023 at 04:39:37PM -0400, Jason Gunthorpe wrote:
> > On Thu, Feb 09, 2023 at 11:51:31AM -0800, Nicolin Chen wrote:
> > > On Thu, Feb 09, 2023 at 02:36:49PM -0400, Jason Gunthorpe wrote:
> > > > On Thu, Feb 09, 2023 at 12:59:58PM -0500, Matthew Rosato wrote:
> > > > > really should highlight that).  Otherwise, conditionally calling
> > > > > iommu_domain_alloc(dev->bus) when !ops->domain_alloc_user
> (instead
> > > > > of returning -EOPNOTSUPP) seems to restore the prior functionality
> > > > > for me.
> > > >
> > > > Yes, that is right if the input user data is 0 length or full of 0s
> > > > then we should call the normal driver function
> > >
> > > Maybe I am wrong, yet I recall that doing ops->domain_alloc_user
> > > without a fallback was intentional to rule out drivers that don't
> > > support IOMMUFD?
> >
> > I think we moved away from that to the idea of using the dma_domain
> > patch I suggested..
> >
> > > To be backward-compatible and concern about SMMU, we can opt in
> > > ops->domain_alloc_user upon availability and then add a fallback:
> > >
> > > 	if ((!ops || !ops->domain_alloc_user) && user_data) {
> > > 		rc = -EOPNOTSUPP;
> > > 		goto out_abort;
> > > 	}
> > >
> > > 	if (ops->domain_alloc_user)
> > > 		hwpt->domain = ops->domain_alloc_user(dev, NULL, NULL);
> > > 	else
> > > 		hwpt->domain = iommu_domain_alloc(dev->bus);
> > > 	if (!hwpt->domain) {
> > > 		rc = -ENOMEM;
> > > 		goto out_abort;
> > > 	}
> > >
> > > Yet, even by doing so, this series having the PATCH 07/17 that
> > > moves iopt_table_add_domain() would temporally break IOMMUFD on
> > > ARM platforms, until we add the ops->domain_alloc_user to SMMU
> > > drivers.
> >
> > Drop patch 7 and 8
> >
> > Change patch 12 so it has a unique flow to allocate and IOAS map a
> > HWPT that does not try to share so much code with the existing flow.
> >
> > The HWPT flow should always just call allocate and then map with no
> > effort to attach first. This will fail on ARM SMMU at this point, and
> > that is fine.

Ok. this sounds iopt_table_add_domain() should still be called
right after user hwpt is allocated instead of first device attached.

> >
> > All the existing code should work exactly as it is now and not have
> > any need to be changed.
> >
> > Where things when wrong was trying to share
> > "__iommufd_hw_pagetable_alloc", I think.
> >

Sure. 

> > Don't try to consolidate at this point. Once all the drivers are
> > updated then we could try to consolidate things.
> 
> Yea, I think that's the only way out for now. Though I am not
> sure about other drivers yet, hopefully the SMMU driver(s) is
> the last one that we need to update...

😊 hope it.

Regards,
Yi Liu