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

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Message-ID: <e6ba7689-92a9-e332-d364-24e324bdad38@linux.intel.com>
Date:   Tue, 17 Dec 2019 12:13:35 +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:     "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,

On 2019/12/17 10:36, Liu, Yi L wrote:
>> From: Liu, Yi L <yi.l.liu@...el.com>
>> Sent: Tuesday, December 17, 2019 10:26 AM
>> To: Lu Baolu <baolu.lu@...ux.intel.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
>>
>>> 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.
> 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.

Sure. I will.

Best regards,

baolu
>   
> Regards,
> Yi Liu