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Re: [PATCH v9 Kernel 2/5] vfio iommu: Add ioctl defination to get dirty
From: |
Alex Williamson |
Subject: |
Re: [PATCH v9 Kernel 2/5] vfio iommu: Add ioctl defination to get dirty pages bitmap. |
Date: |
Wed, 4 Dec 2019 23:40:12 -0700 |
On Thu, 5 Dec 2019 11:49:00 +0530
Kirti Wankhede <address@hidden> wrote:
> On 12/5/2019 11:26 AM, Alex Williamson wrote:
> > On Thu, 5 Dec 2019 11:12:23 +0530
> > Kirti Wankhede <address@hidden> wrote:
> >
> >> 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
> >>>>>>>>>>>>
> >>>>>>>>>>>> definition
> >>>>>>>>>>>>
> >>>>>>>>>>>>> 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 pages,
> >>>>>>> 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,
> >>>>
> >>>> Alex
> >>>>
> >>>> * 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.
> >
> > We can't rely on the vCPU also dirtying a page, the overhead is
> > unavoidable. It doesn't matter if the migration is fast if it's
> > incorrect. We only need to track unpinned dirty pages while the
> > migration is active and the tracking is flushed on each log_sync
> > callback. Thanks,
> >
>
> From Yan's comment, pasted below, I thought, need to track all unpinned
> pages during application or VM's lifetime.
>
> > There we should also be aware of that dirty page scope is pinned
> pages > + unpinned pages, which means ever since a page is pinned, it
> should
> > be regarded as dirty no matter whether it's unpinned later.
>
> But if its about tracking pages which are unpinned "while the migration
> is active", then that set would be less, will do this change.
When we first start a pre-copy, all RAM (including anything previously
pinned) is dirty anyway. I believe the log_sync callback is only
intended to report pages dirtied since the migration was started, or
since the last log_sync callback. We then assume that currently pinned
pages are constantly dirty and previously pinned pages are dirty until
we've reported them through log_sync. Thanks,
Alex
- Re: [PATCH v9 Kernel 2/5] vfio iommu: Add ioctl defination to get dirty pages bitmap., Alex Williamson, 2019/12/03
- Re: [PATCH v9 Kernel 2/5] vfio iommu: Add ioctl defination to get dirty pages bitmap., Kirti Wankhede, 2019/12/04
- Re: [PATCH v9 Kernel 2/5] vfio iommu: Add ioctl defination to get dirty pages bitmap., Alex Williamson, 2019/12/04
- Re: [PATCH v9 Kernel 2/5] vfio iommu: Add ioctl defination to get dirty pages bitmap., Yan Zhao, 2019/12/04
- Re: [PATCH v9 Kernel 2/5] vfio iommu: Add ioctl defination to get dirty pages bitmap., Kirti Wankhede, 2019/12/05
- Re: [PATCH v9 Kernel 2/5] vfio iommu: Add ioctl defination to get dirty pages bitmap., Yan Zhao, 2019/12/05
- Re: [PATCH v9 Kernel 2/5] vfio iommu: Add ioctl defination to get dirty pages bitmap., Alex Williamson, 2019/12/05
- Re: [PATCH v9 Kernel 2/5] vfio iommu: Add ioctl defination to get dirty pages bitmap., Kirti Wankhede, 2019/12/05
- Re: [PATCH v9 Kernel 2/5] vfio iommu: Add ioctl defination to get dirty pages bitmap.,
Alex Williamson <=