Re: [PATCH RFC v1 13/15] iommu/virtio: Attach Arm PASID tables when available

[Jean-Philippe Brucker]

On Fri, Mar 12, 2021 at 06:59:17PM +0530, Vivek Kumar Gautam wrote:
> > > +	/* XXX HACK: set feature bit ARM_SMMU_FEAT_2_LVL_CDTAB */
> > > +	pst_cfg->vendor.cfg.feat_flag |= (1 << 1);
> > 
> > Oh right, this flag is missing. I'll add
> > 
> >    #define VIRTIO_IOMMU_PST_ARM_SMMU3_F_CD2L (1ULL << 1)
> > 
> > to the spec.
> 
> Regarding this Eric pointed out [1] in my other patch about the scalability
> of the approach where we keep adding flags in 'iommu_nesting_info'
> corresponding to the arm-smmu-v3 capabilities. I guess the same goes to
> these flags in virtio.
> May be the 'iommu_nesting_info' can have a bitmap with the caps for vendor
> specific features, and here we can add the related flags?

Something like that, but I'd keep separate arch-specific structs. Vt-d
reports the capability registers directly through iommu_nesting_info [2].
We could do the same for Arm, copy sanitized values of IDR0..5 into
struct iommu_nesting_info_arm_smmuv3.

I've avoided doing that for virtio-iommu because every field needs a
description in the spec. So where possible I used generic properties that
apply to any architecture, such as page, PASID and address size. What's
left is the minimum arch-specific information to get nested translation
going, leaving out a lot of properties such as big-endian and 32-bit,
which can be added later if needed. The Arm specific properties are split
into page table and pasid table information. Page table info should work
for both SMMUv2 and v3 (where they correspond to an SMMU_IDRx field that
constrains a context descriptor field.) I should move BTM in there since
it's supported by SMMUv2.

Thanks,
Jean

[2] https://lore.kernel.org/linux-iommu/20210302203545.436623-11-yi.l.liu@xxxxxxxxx/