On Thu, Dec 05, 2019 at 01:42:23PM +0800, Kirti Wankhede 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 <kwankhede@xxxxxxxxxx> wrote: > >> > >>> On 12/3/2019 11:34 PM, Alex Williamson wrote: > >>>> On Mon, 25 Nov 2019 19:57:39 -0500 > >>>> Yan Zhao <yan.y.zhao@xxxxxxxxx> 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 <kwankhede@xxxxxxxxxx> wrote: > >>>>>> > >>>>>>> On 11/14/2019 1:37 AM, Alex Williamson wrote: > >>>>>>>> On Thu, 14 Nov 2019 01:07:21 +0530 > >>>>>>>> Kirti Wankhede <kwankhede@xxxxxxxxxx> wrote: > >>>>>>>> > >>>>>>>>> On 11/13/2019 4:00 AM, Alex Williamson wrote: > >>>>>>>>>> On Tue, 12 Nov 2019 22:33:37 +0530 > >>>>>>>>>> Kirti Wankhede <kwankhede@xxxxxxxxxx> 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. > > Thanks, > Kirti Those are pages dirtied by vGPU during Pin/Unpin. They are not dirtied by vCPUs. RAM module in QEMU has no idea of it.
