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 09:37:22 +0800
From:   Lu Baolu <baolu.lu@...ux.intel.com>
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>
Cc:     baolu.lu@...ux.intel.com, "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

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?

> 
>> 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!

Best regards,
baolu