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

[Alex Williamson <alex.williamson@redhat.com>]

On Thu, 5 Dec 2019 11:49:00 +0530
Kirti Wankhede <kwankhede@nvidia.com> wrote:

> On 12/5/2019 11:26 AM, Alex Williamson wrote:
> > On Thu, 5 Dec 2019 11:12:23 +0530
> > Kirti Wankhede <kwankhede@nvidia.com> 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@nvidia.com> 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@intel.com> 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@nvidia.com> wrote:
> >>>>>>>>            
> >>>>>>>>> On 11/14/2019 1:37 AM, Alex Williamson wrote:  
> >>>>>>>>>> On Thu, 14 Nov 2019 01:07:21 +0530
> >>>>>>>>>> Kirti Wankhede <kwankhede@nvidia.com> wrote:
> >>>>>>>>>>              
> >>>>>>>>>>> On 11/13/2019 4:00 AM, Alex Williamson wrote:  
> >>>>>>>>>>>> On Tue, 12 Nov 2019 22:33:37 +0530
> >>>>>>>>>>>> Kirti Wankhede <kwankhede@nvidia.com> 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