Re: [PATCH v7 03/11] iommu/vt-d: Add custom allocator for IOASID

[Lu Baolu]

Hi,

On 10/25/19 2:40 PM, Tian, Kevin wrote:
> > > > ioasid_register_allocator(&iommu->pasid_allocator);
> > > > +			if (ret) {
> > > > +				pr_warn("Custom PASID allocator
> > > > registeration failed\n");
> > > > +				/*
> > > > +				 * Disable scalable mode on this
> > > > IOMMU if there
> > > > +				 * is no custom allocator. Mixing
> > > > SM capable vIOMMU
> > > > +				 * and non-SM vIOMMU are not
> > > > supported.
> > > > +				 */
> > > > +				intel_iommu_sm = 0;
> > > It's insufficient to disable scalable mode by only clearing
> > > intel_iommu_sm. The DMA_RTADDR_SMT bit in root entry has already
> > been
> > > set. Probably, you need to
> > >
> > > for each iommu
> > > 	clear DMA_RTADDR_SMT in root entry
> > >
> > > Alternatively, since vSVA is the only customer of this custom PASID
> > > allocator, is it possible to only disable SVA here?
> > Yeah, I think disable SVA is better. We can still do gIOVA in SM. I
> > guess we need to introduce a flag for sva_enabled.
> I'm not sure whether tying above logic to SVA is the right approach.
> If vcmd interface doesn't work, the whole SM mode doesn't make
> sense which is based on PASID-granular protection (SVA is only one
> usage atop). If the only remaining usage of SM is to map gIOVA using
> reserved PASID#0, then why not disabling SM and just fallback to
> legacy mode?
>
> Based on that I prefer to disabling the SM mode completely (better
> through an interface), and move the logic out of CONFIG_INTEL_
> IOMMU_SVM

Unfortunately, it is dangerous to disable SM after boot. SM uses
different root/device contexts and pasid table formats. Disabling SM
after boot requires changing above from SM format into legacy format.

Since ioasid registration failure is a rare case. How about moving this
part of code up to the early stage of intel_iommu_init() and returning
error if hardware present vcmd capability but software fails to register
a custom ioasid allocator?

Best regards,
baolu