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Re: [PATCH v15 Kernel 4/7] vfio iommu: Implementation of ioctl for dirty

From: Kirti Wankhede
Subject: Re: [PATCH v15 Kernel 4/7] vfio iommu: Implementation of ioctl for dirty pages tracking.
Date: Mon, 23 Mar 2020 23:24:37 +0530
User-agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:68.0) Gecko/20100101 Thunderbird/68.6.0

On 3/21/2020 12:29 AM, Alex Williamson wrote:
On Sat, 21 Mar 2020 00:12:04 +0530
Kirti Wankhede <address@hidden> wrote:

On 3/20/2020 11:31 PM, Alex Williamson wrote:
On Fri, 20 Mar 2020 23:19:14 +0530
Kirti Wankhede <address@hidden> wrote:
On 3/20/2020 4:27 AM, Alex Williamson wrote:
On Fri, 20 Mar 2020 01:46:41 +0530
Kirti Wankhede <address@hidden> wrote:


+static int vfio_iova_dirty_bitmap(struct vfio_iommu *iommu, dma_addr_t iova,
+                                 size_t size, uint64_t pgsize,
+                                 u64 __user *bitmap)
+       struct vfio_dma *dma;
+       unsigned long pgshift = __ffs(pgsize);
+       unsigned int npages, bitmap_size;
+       dma = vfio_find_dma(iommu, iova, 1);
+       if (!dma)
+               return -EINVAL;
+       if (dma->iova != iova || dma->size != size)
+               return -EINVAL;
+       npages = dma->size >> pgshift;
+       bitmap_size = DIRTY_BITMAP_BYTES(npages);
+       /* mark all pages dirty if all pages are pinned and mapped. */
+       if (dma->iommu_mapped)
+               bitmap_set(dma->bitmap, 0, npages);
+       if (copy_to_user((void __user *)bitmap, dma->bitmap, bitmap_size))
+               return -EFAULT;

We still need to reset the bitmap here, clearing and re-adding the
pages that are still pinned.


I thought you agreed on my reply to it
> Why re-populate when there will be no change since
   > vfio_iova_dirty_bitmap() is called holding iommu->lock? If there is any
   > pin request while vfio_iova_dirty_bitmap() is still working, it will
   > wait till iommu->lock is released. Bitmap will be populated when page is
   > pinned.

As coded, dirty bits are only ever set in the bitmap, never cleared.
If a page is unpinned between iterations of the user recording the
dirty bitmap, it should be marked dirty in the iteration immediately
after the unpinning and not marked dirty in the following iteration.
That doesn't happen here.  We're reporting cumulative dirty pages since
logging was enabled, we need to be reporting dirty pages since the user
last retrieved the dirty bitmap.  The bitmap should be cleared and
currently pinned pages re-added after copying to the user.  Thanks,

Does that mean, we have to track every iteration? do we really need that

Generally the flow is:
- vendor driver pin x pages
- Enter pre-copy-phase where vCPUs are running - user starts dirty pages
tracking, then user asks dirty bitmap, x pages reported dirty by
- In pre-copy phase, vendor driver pins y more pages, now bitmap
consists of x+y bits set
- In pre-copy phase, vendor driver unpins z pages, but bitmap is not
updated, so again bitmap consists of x+y bits set.
- Enter in stop-and-copy phase, vCPUs are stopped, mdev devices are stopped
- user asks dirty bitmap - Since here vCPU and mdev devices are stopped,
pages should not get dirty by guest driver or the physical device.
Hence, x+y dirty pages would be reported.

I don't think we need to track every iteration of bitmap reporting.

Yes, once a bitmap is read, it's reset.  In your example, after
unpinning z pages the user should still see a bitmap with x+y pages,
but once they've read that bitmap, the next bitmap should be x+y-z.
Userspace can make decisions about when to switch from pre-copy to
stop-and-copy based on convergence, ie. the slope of the line recording
dirty pages per iteration.  The implementation here never allows an
inflection point, dirty pages reported through vfio would always either
be flat or climbing.  There might also be a case that an iommu backed
device could start pinning pages during the course of a migration, how
would the bitmap ever revert from fully populated to only tracking the
pinned pages?  Thanks,

At KVM forum we discussed this - if guest driver pins say 1024 pages before migration starts, during pre-copy phase device can dirty 0 pages in best case and 1024 pages in worst case. In that case, user will transfer content of 1024 pages during pre-copy phase and in stop-and-copy phase also, that will be pages will be copied twice. So we decided to only get dirty pages bitmap at stop-and-copy phase. If user is going to get dirty pages in stop-and-copy phase only, then that will be single iteration. There aren't any devices yet that can track sys memory dirty pages. So we can go ahead with this patch and support for dirty pages tracking during pre-copy phase can be added later when there will be consumers of that functionality.


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