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

[Lu Baolu]

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