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Re: [PATCH v9 Kernel 2/5] vfio iommu: Add ioctl defination to get dirty

From: Kirti Wankhede
Subject: Re: [PATCH v9 Kernel 2/5] vfio iommu: Add ioctl defination to get dirty pages bitmap.
Date: Thu, 5 Dec 2019 11:12:23 +0530
User-agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:68.0) Gecko/20100101 Thunderbird/68.1.2

On 12/5/2019 6:58 AM, Yan Zhao wrote:
On Thu, Dec 05, 2019 at 02:34:57AM +0800, Alex Williamson wrote:
On Wed, 4 Dec 2019 23:40:25 +0530
Kirti Wankhede <address@hidden> wrote:

On 12/3/2019 11:34 PM, Alex Williamson wrote:
On Mon, 25 Nov 2019 19:57:39 -0500
Yan Zhao <address@hidden> wrote:
On Fri, Nov 15, 2019 at 05:06:25AM +0800, Alex Williamson wrote:
On Fri, 15 Nov 2019 00:26:07 +0530
Kirti Wankhede <address@hidden> wrote:
On 11/14/2019 1:37 AM, Alex Williamson wrote:
On Thu, 14 Nov 2019 01:07:21 +0530
Kirti Wankhede <address@hidden> wrote:
On 11/13/2019 4:00 AM, Alex Williamson wrote:
On Tue, 12 Nov 2019 22:33:37 +0530
Kirti Wankhede <address@hidden> wrote:
All pages pinned by vendor driver through vfio_pin_pages API should be
considered as dirty during migration. IOMMU container maintains a list of
all such pinned pages. Added an ioctl defination to get bitmap of such

pinned pages for requested IO virtual address range.

Additionally, all mapped pages are considered dirty when physically
mapped through to an IOMMU, modulo we discussed devices opting in to
per page pinning to indicate finer granularity with a TBD mechanism to
figure out if any non-opt-in devices remain.

You mean, in case of device direct assignment (device pass through)?

Yes, or IOMMU backed mdevs.  If vfio_dmas in the container are fully
pinned and mapped, then the correct dirty page set is all mapped pages.
We discussed using the vpfn list as a mechanism for vendor drivers to
reduce their migration footprint, but we also discussed that we would
need a way to determine that all participants in the container have
explicitly pinned their working pages or else we must consider the
entire potential working set as dirty.

How can vendor driver tell this capability to iommu module? Any suggestions?

I think it does so by pinning pages.  Is it acceptable that if the
vendor driver pins any pages, then from that point forward we consider
the IOMMU group dirty page scope to be limited to pinned pages?  There
we should also be aware of that dirty page scope is pinned pages + unpinned 
which means ever since a page is pinned, it should be regarded as dirty
no matter whether it's unpinned later. only after log_sync is called and
dirty info retrieved, its dirty state should be cleared.

Yes, good point.  We can't just remove a vpfn when a page is unpinned
or else we'd lose information that the page potentially had been
dirtied while it was pinned.  Maybe that vpfn needs to move to a dirty
list and both the currently pinned vpfns and the dirty vpfns are walked
on a log_sync.  The dirty vpfns list would be cleared after a log_sync.
The container would need to know that dirty tracking is enabled and
only manage the dirty vpfns list when necessary.  Thanks,

If page is unpinned, then that page is available in free page pool for
others to use, then how can we say that unpinned page has valid data?

If suppose, one driver A unpins a page and when driver B of some other
device gets that page and he pins it, uses it, and then unpins it, then
how can we say that page has valid data for driver A?

Can you give one example where unpinned page data is considered reliable
and valid?

We can only pin pages that the user has already allocated* and mapped
through the vfio DMA API.  The pinning of the page simply locks the
page for the vendor driver to access it and unpinning that page only
indicates that access is complete.  Pages are not freed when a vendor
driver unpins them, they still exist and at this point we're now
assuming the device dirtied the page while it was pinned.  Thanks,


* An exception here is that the page might be demand allocated and the
   act of pinning the page could actually allocate the backing page for
   the user if they have not faulted the page to trigger that allocation
   previously.  That page remains mapped for the user's virtual address
   space even after the unpinning though.

Yes, I can give an example in GVT.
when a gem_object is allocated in guest, before submitting it to guest
vGPU, gfx cmds in its ring buffer need to be pinned into GGTT to get a
global graphics address for hardware access. At that time, we shadow
those cmds and pin pages through vfio pin_pages(), and submit the shadow
gem_object to physial hardware.
After guest driver thinks the submitted gem_object has completed hardware
DMA, it unnpinnd those pinned GGTT graphics memory addresses. Then in
host, we unpin the shadow pages through vfio unpin_pages.
But, at this point, guest driver is still free to access the gem_object
through vCPUs, and guest user space is probably still mapping an object
into the gem_object in guest driver.
So, missing the dirty page tracking for unpinned pages would cause
data inconsitency.

If pages are accessed by guest through vCPUs, then RAM module in QEMU will take care of tracking those pages as dirty.

All unpinned pages might not be used, so tracking all unpinned pages during VM or application life time would also lead to tracking lots of stale pages, even though they are not being used. Increasing number of not needed pages could also lead to increasing migration data leading increase in migration downtime.


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