linux-kernel - Re: [RFC PATCH v4 01/13] iommu: Introduce dirty log tracking framework

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Date:   Tue, 11 May 2021 11:12:07 +0800
From:   Lu Baolu <baolu.lu@...ux.intel.com>
To:     Keqian Zhu <zhukeqian1@...wei.com>, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        iommu@...ts.linux-foundation.org,
        Robin Murphy <robin.murphy@....com>,
        Will Deacon <will@...nel.org>, Joerg Roedel <joro@...tes.org>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        Yi Sun <yi.y.sun@...ux.intel.com>,
        Tian Kevin <kevin.tian@...el.com>
Cc:     baolu.lu@...ux.intel.com,
        Alex Williamson <alex.williamson@...hat.com>,
        Kirti Wankhede <kwankhede@...dia.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        wanghaibin.wang@...wei.com, jiangkunkun@...wei.com,
        yuzenghui@...wei.com, lushenming@...wei.com
Subject: Re: [RFC PATCH v4 01/13] iommu: Introduce dirty log tracking
 framework

Hi Keqian,

On 5/10/21 7:07 PM, Keqian Zhu wrote:
>>>> I suppose this interface is to ask the vendor IOMMU driver to check
>>>> whether each device/iommu in the domain supports dirty bit tracking.
>>>> But what will happen if new devices with different tracking capability
>>>> are added afterward?
>>> Yep, this is considered in the vfio part. We will query again after attaching or
>>> detaching devices from the domain.  When the domain becomes capable, we enable
>>> dirty log for it. When it becomes not capable, we disable dirty log for it.
>> If that's the case, why not putting this logic in the iommu subsystem so
>> that it doesn't need to be duplicate in different upper layers?
>>
>> For example, add something like dirty_page_trackable in the struct of
>> iommu_domain and ask the vendor iommu driver to update it once any
>> device is added/removed to/from the domain. It's also better to disallow
> If we do it, the upper layer still needs to query the capability from domain and switch
> dirty log tracking for it. Or do you mean the domain can switch dirty log tracking automatically
> when its capability change? If so, I think we're lack of some flexibility. The upper layer
> may have it's own policy, such as only enable dirty log tracking when all domains are capable,
> and disable dirty log tracking when just one domain is not capable.

I may not get you.

Assume that dirty_page_trackable is an attribution of an iommu_domain.
This attribution might be changed once a new device (with different
capability) added or removed. So it should be updated every time a new
device is attached or detached. This work could be done by the vendor
iommu driver on the path of dev_attach/dev_detach callback.

For upper layers, before starting page tracking, they check the
dirty_page_trackable attribution of the domain and start it only it's
capable. Once the page tracking is switched on the vendor iommu driver
(or iommu core) should block further device attach/detach operations
until page tracking is stopped.

> 
>> any domain attach/detach once the dirty page tracking is on.
> Yep, this can greatly simplify our code logic, but I don't know whether our maintainers
> agree that, as they may think that IOMMU dirty logging should not change original domain
> behaviors.

The maintainer owns the last word, but we need to work out a generic and
self-contained API set.

Best regards,
baolu