RE: [PATCH v5 03/15] iommu/smmu: Report empty domain nesting info

[Liu, Yi L]

Hi Jean,

> From: Jean-Philippe Brucker <jean-philippe@xxxxxxxxxx>
> Sent: Thursday, July 16, 2020 11:40 PM
> 
> On Tue, Jul 14, 2020 at 10:12:49AM +0000, Liu, Yi L wrote:
> > > Have you verified that this doesn't break the existing usage of
> > > DOMAIN_ATTR_NESTING in drivers/vfio/vfio_iommu_type1.c?
> >
> > I didn't have ARM machine on my hand. But I contacted with Jean
> > Philippe, he confirmed no compiling issue. I didn't see any code
> > getting DOMAIN_ATTR_NESTING attr in current drivers/vfio/vfio_iommu_type1.c.
> > What I'm adding is to call iommu_domai_get_attr(, DOMAIN_ATTR_NESTIN)
> > and won't fail if the iommu_domai_get_attr() returns 0. This patch
> > returns an empty nesting info for DOMAIN_ATTR_NESTIN and return
> > value is 0 if no error. So I guess it won't fail nesting for ARM.
> 
> I confirm that this series doesn't break the current support for
> VFIO_IOMMU_TYPE1_NESTING with an SMMUv3. That said...

thanks.

> If the SMMU does not support stage-2 then there is a change in behavior
> (untested): after the domain is silently switched to stage-1 by the SMMU
> driver, VFIO will now query nesting info and obtain -ENODEV. Instead of
> succeding as before, the VFIO ioctl will now fail. I believe that's a fix
> rather than a regression, it should have been like this since the
> beginning. No known userspace has been using VFIO_IOMMU_TYPE1_NESTING so
> far, so I don't think it should be a concern.
> 
> And if userspace queries the nesting properties using the new ABI
> introduced in this patchset, it will obtain an empty struct.

yes.

> I think
> that's acceptable, but it may be better to avoid adding the nesting cap if
> @format is 0?

right. will add it in patch 4/15.

Regards,
Yi Liu

> 
> Thanks,
> Jean
> 
> >
> > @Eric, how about your opinion? your dual-stage vSMMU support may
> > also share the vfio_iommu_type1.c code.
> >
> > Regards,
> > Yi Liu
> >
> > > Will