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

Jacob Pan
2019-10-28T22:44:39Z

On Fri, 25 Oct 2019 15:52:39 +0000
"Tian, Kevin" <kevin.tian@intel.com> wrote:

> > From: Lu Baolu [mailto:baolu.lu@linux.intel.com]
> > Sent: Friday, October 25, 2019 10:39 PM
> >
> > 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.
>
> You are correct.
>
> >
> > 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?
> >
>
> It makes sense to me.
>
sounds good to me too, the earlier the less to clean up.
> Thanks
> Kevin

[Jacob Pan]