RE: [PATCH v3 5/6] iommu/vt-d: Flush PASID-based iotlb for iova over first level

Liu, Yi L
2019-12-17T02:36:33Z

> From: Liu, Yi L <yi.l.liu@intel.com>
> Sent: Tuesday, December 17, 2019 10:26 AM
> To: Lu Baolu <baolu.lu@linux.intel.com>; Joerg Roedel <joro@8bytes.org>; David
> Woodhouse <dwmw2@infradead.org>; Alex Williamson
> <alex.williamson@redhat.com>
> Subject: RE: [PATCH v3 5/6] iommu/vt-d: Flush PASID-based iotlb for iova over first
> level
>
> > From: Lu Baolu [mailto:baolu.lu@linux.intel.com]
> > Sent: Tuesday, December 17, 2019 9:37 AM
> > To: Liu, Yi L <yi.l.liu@intel.com>; Joerg Roedel <joro@8bytes.org>; David
> > Woodhouse <dwmw2@infradead.org>; Alex Williamson
> > <alex.williamson@redhat.com>
> > Subject: Re: [PATCH v3 5/6] iommu/vt-d: Flush PASID-based iotlb for iova over first
> > level
> >
> > Hi again,
> >
> > On 12/17/19 9:19 AM, Lu Baolu wrote:
> > > Hi Yi,
> > >
> > > On 12/15/19 5:22 PM, Liu, Yi L wrote:
> > >> Ok, let me explain more... default pasid is meaningful only when
> > >> the domain has been attached to a device as an aux-domain. right?
> > >
> > > No exactly. Each domain has a specific default pasid, no matter normal
> > > domain (RID based) or aux-domain (PASID based). The difference is for a
> > > normal domain RID2PASID value is used, for an aux-domain the pasid is
> > > allocated from a global pool.
> > >
> > > The same concept used in VT-d 3.x scalable mode. For RID based DMA
> > > translation RID2PASID value is used when walking the tables; For PASID
> > > based DMA translation a real pasid in the transaction is used.
> > >
> > >> If a domain only has one device, and it is attached to this device as
> > >> normal domain (normal domain means non aux-domain here). Then
> > >> you should flush cache with domain-id and RID2PASID value.
> > >> If a domain has one device, and it is attached to this device as
> > >> aux-domain. Then you may want to flush cache with domain-id
> > >> and default pasid. right?
> > >
> > > A domain's counterpart is IOMMU group. So we say attach/detach domain
> > > to/from devices in a group. We don't allow devices with different
> > > default pasid sitting in a same group, right?
> > >
> > >> Then let's come to the case I mentioned in previous email. a mdev
> > >> and another device assigned to a single VM. In host, you will have
> > >> a domain which has two devices, one device(deva) is attached as
> > >
> > > No. We will have two IOMMU groups and two domains. Correct me if my
> > > understanding is not right.
> >
> > Reconsidered this. Unfortunately, my understanding is not right. :-(
> >
> > A single domain could be attached to multiple IOMMU groups. So it
> > comes to the issue you concerned. Do I understand it right?
>
> yes. Device within the same group has no such issue since such
> devices are not able to enabled aux-domain. Now our understanding
> are aligned. :-)
>
> > >
> > >> normal domain, another one (devB) is attached as aux-domain. Then
> > >> which pasid should be used when the mapping in IOVA page table is
> > >> modified? RID2PASID or default pasid? I think both should be used
> > >> since the domain means differently to the two devices. If you just
> > >> use default pasid, then deva may still be able to use stale caches.
> >
> > You are right. I will change it accordingly. The logic should look
> > like:
> >
> > if (domain attached to physical device)
> > flush_piotlb_with_RID2PASID()
> > else if (domain_attached_to_mdev_device)
> > flush_piotlb_with_default_pasid()
> >
> > Does this work for you? Thanks for catching this!
>
> If no else, it would work for scalable mode. ^_^ I noticed you've
> already corrected by yourself in another reply. :-) Look forward to
> your next version.

BTW. The discussion in this thread may apply to other cache flush
in your series. Please have a check. At least, there are two places which
need to be updated in this single patch.

Regards,
Yi Liu