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

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Date:   Tue, 17 Dec 2019 02:26:17 +0000
From:   "Liu, Yi L" <yi.l.liu@...el.com>
To:     Lu Baolu <baolu.lu@...ux.intel.com>,
        Joerg Roedel <joro@...tes.org>,
        David Woodhouse <dwmw2@...radead.org>,
        Alex Williamson <alex.williamson@...hat.com>
CC:     "Raj, Ashok" <ashok.raj@...el.com>,
        "Kumar, Sanjay K" <sanjay.k.kumar@...el.com>,
        "jacob.jun.pan@...ux.intel.com" <jacob.jun.pan@...ux.intel.com>,
        "Tian, Kevin" <kevin.tian@...el.com>,
        "Sun, Yi Y" <yi.y.sun@...el.com>, Peter Xu <peterx@...hat.com>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v3 5/6] iommu/vt-d: Flush PASID-based iotlb for iova
 over first level

> From: Lu Baolu [mailto:baolu.lu@...ux.intel.com]
> Sent: Tuesday, December 17, 2019 9:37 AM
> To: Liu, Yi L <yi.l.liu@...el.com>; Joerg Roedel <joro@...tes.org>; David
> Woodhouse <dwmw2@...radead.org>; Alex Williamson
> <alex.williamson@...hat.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.

Regards,
Yi Liu