Re: [PATCH Kernel v20 6/8] vfio iommu: Update UNMAP_DMA ioctl to get dirty bitmap before unmap

[Alex Williamson]

On Fri, 15 May 2020 09:46:43 +0530
Kirti Wankhede <address@hidden> wrote:

> On 5/15/2020 8:57 AM, Alex Williamson wrote:
> > On Fri, 15 May 2020 02:07:45 +0530
> > Kirti Wankhede <address@hidden> wrote:
> >   
> >> DMA mapped pages, including those pinned by mdev vendor drivers, might
> >> get unpinned and unmapped while migration is active and device is still
> >> running. For example, in pre-copy phase while guest driver could access
> >> those pages, host device or vendor driver can dirty these mapped pages.
> >> Such pages should be marked dirty so as to maintain memory consistency
> >> for a user making use of dirty page tracking.
> >>
> >> To get bitmap during unmap, user should allocate memory for bitmap, set
> >> it all zeros, set size of allocated memory, set page size to be
> >> considered for bitmap and set flag VFIO_DMA_UNMAP_FLAG_GET_DIRTY_BITMAP.
> >>
> >> Signed-off-by: Kirti Wankhede <address@hidden>
> >> Reviewed-by: Neo Jia <address@hidden>
> >> ---
> >>   drivers/vfio/vfio_iommu_type1.c | 77 
> >> ++++++++++++++++++++++++++++++++++-------
> >>   include/uapi/linux/vfio.h       | 10 ++++++
> >>   2 files changed, 75 insertions(+), 12 deletions(-)
> >>
> >> diff --git a/drivers/vfio/vfio_iommu_type1.c 
> >> b/drivers/vfio/vfio_iommu_type1.c
> >> index b76d3b14abfd..a1dc57bcece5 100644
> >> --- a/drivers/vfio/vfio_iommu_type1.c
> >> +++ b/drivers/vfio/vfio_iommu_type1.c
> >> @@ -195,11 +195,15 @@ static void vfio_unlink_dma(struct vfio_iommu 
> >> *iommu, struct vfio_dma *old)
> >>   static int vfio_dma_bitmap_alloc(struct vfio_dma *dma, size_t pgsize)
> >>   {
> >>    uint64_t npages = dma->size / pgsize;
> >> +  size_t bitmap_size;
> >>   
> >>    if (npages > DIRTY_BITMAP_PAGES_MAX)
> >>            return -EINVAL;
> >>   
> >> -  dma->bitmap = kvzalloc(DIRTY_BITMAP_BYTES(npages), GFP_KERNEL);
> >> +  /* Allocate extra 64 bits which are used for bitmap manipulation */
> >> +  bitmap_size = DIRTY_BITMAP_BYTES(npages) + sizeof(u64);
> >> +
> >> +  dma->bitmap = kvzalloc(bitmap_size, GFP_KERNEL);
> >>    if (!dma->bitmap)
> >>            return -ENOMEM;
> >>   
> >> @@ -999,23 +1003,25 @@ static int verify_bitmap_size(uint64_t npages, 
> >> uint64_t bitmap_size)
> >>   }
> >>   
> >>   static int vfio_dma_do_unmap(struct vfio_iommu *iommu,
> >> -                       struct vfio_iommu_type1_dma_unmap *unmap)
> >> +                       struct vfio_iommu_type1_dma_unmap *unmap,
> >> +                       struct vfio_bitmap *bitmap)
> >>   {
> >> -  uint64_t mask;
> >>    struct vfio_dma *dma, *dma_last = NULL;
> >> -  size_t unmapped = 0;
> >> +  size_t unmapped = 0, pgsize;
> >>    int ret = 0, retries = 0;
> >> +  unsigned long pgshift;
> >>   
> >>    mutex_lock(&iommu->lock);
> >>   
> >> -  mask = ((uint64_t)1 << __ffs(iommu->pgsize_bitmap)) - 1;
> >> +  pgshift = __ffs(iommu->pgsize_bitmap);
> >> +  pgsize = (size_t)1 << pgshift;
> >>   
> >> -  if (unmap->iova & mask) {
> >> +  if (unmap->iova & (pgsize - 1)) {
> >>            ret = -EINVAL;
> >>            goto unlock;
> >>    }
> >>   
> >> -  if (!unmap->size || unmap->size & mask) {
> >> +  if (!unmap->size || unmap->size & (pgsize - 1)) {
> >>            ret = -EINVAL;
> >>            goto unlock;
> >>    }
> >> @@ -1026,9 +1032,15 @@ static int vfio_dma_do_unmap(struct vfio_iommu 
> >> *iommu,
> >>            goto unlock;
> >>    }
> >>   
> >> -  WARN_ON(mask & PAGE_MASK);
> >> -again:
> >> +  /* When dirty tracking is enabled, allow only min supported pgsize */
> >> +  if ((unmap->flags & VFIO_DMA_UNMAP_FLAG_GET_DIRTY_BITMAP) &&
> >> +      (!iommu->dirty_page_tracking || (bitmap->pgsize != pgsize))) {
> >> +          ret = -EINVAL;
> >> +          goto unlock;
> >> +  }
> >>   
> >> +  WARN_ON((pgsize - 1) & PAGE_MASK);
> >> +again:
> >>    /*
> >>     * vfio-iommu-type1 (v1) - User mappings were coalesced together to
> >>     * avoid tracking individual mappings.  This means that the granularity
> >> @@ -1066,6 +1078,7 @@ static int vfio_dma_do_unmap(struct vfio_iommu 
> >> *iommu,
> >>                    ret = -EINVAL;
> >>                    goto unlock;
> >>            }
> >> +
> >>            dma = vfio_find_dma(iommu, unmap->iova + unmap->size - 1, 0);
> >>            if (dma && dma->iova + dma->size != unmap->iova + unmap->size) {
> >>                    ret = -EINVAL;
> >> @@ -1083,6 +1096,23 @@ static int vfio_dma_do_unmap(struct vfio_iommu 
> >> *iommu,
> >>            if (dma->task->mm != current->mm)
> >>                    break;
> >>   
> >> +          if ((unmap->flags & VFIO_DMA_UNMAP_FLAG_GET_DIRTY_BITMAP) &&
> >> +              (dma_last != dma)) {
> >> +
> >> +                  /*
> >> +                   * mark all pages dirty if all pages are pinned and
> >> +                   * mapped
> >> +                   */
> >> +                  if (dma->iommu_mapped)
> >> +                          bitmap_set(dma->bitmap, 0,
> >> +                                     dma->size >> pgshift);  
> > 
> > Nit, all the callers of update_user_bitmap() precede the call with this
> > identical operation, we should probably push it into the function to do
> > it.
> >   
> >> +
> >> +                  ret = update_user_bitmap(bitmap->data, dma,
> >> +                                           unmap->iova, pgsize);
> >> +                  if (ret)
> >> +                          break;
> >> +          }
> >> +  
> > 
> > As noted last time, the above is just busy work if pfn_list is not
> > already empty.  The entire code block above should be moved to after
> > the block below.  Thanks,
> >   
> 
> pfn_list will be empty for IOMMU backed devices where all pages are 
> pinned and mapped,

Unless we're making use of the selective dirtying introduced in patch
8/8 or the container is shared with non-IOMMU backed mdevs.

> but those should be reported as dirty.

I'm confused how that justifies or requires this ordering.

> So moved it 
> back above empty pfn_list check.

Sorry, it still doesn't make any sense to me, and with no discussion I
can't differentiate ignored comments from discarded comments.

Pages in the pfn_list contribute to the dirty bitmap when they're
pinned, we don't depend on pfn_list when reporting the dirty bitmap
except for re-populating pfn_list dirtied pages after the bitmap has
been cleared.  We're unmapping the dma, so that's not the case here.
Also since update_user_bitmap() shifts the bitmap in place now, any
repetitive calls will give us incorrect results.  Therefore, as I see
it, we _can_ take the branch below and when we do any work we've done
above is not only wasted but may lead to incorrect data copied to
the user if we shift dma->bitmap in place more than once.  Please
explain in more detail if you believe this is still correct.  Thanks,

Alex

> >   
> >>            if (!RB_EMPTY_ROOT(&dma->pfn_list)) {
> >>                    struct vfio_iommu_type1_dma_unmap nb_unmap;
> >>   
> >> @@ -2447,17 +2477,40 @@ static long vfio_iommu_type1_ioctl(void 
> >> *iommu_data,
> >>   
> >>    } else if (cmd == VFIO_IOMMU_UNMAP_DMA) {
> >>            struct vfio_iommu_type1_dma_unmap unmap;
> >> -          long ret;
> >> +          struct vfio_bitmap bitmap = { 0 };
> >> +          int ret;
> >>   
> >>            minsz = offsetofend(struct vfio_iommu_type1_dma_unmap, size);
> >>   
> >>            if (copy_from_user(&unmap, (void __user *)arg, minsz))
> >>                    return -EFAULT;
> >>   
> >> -          if (unmap.argsz < minsz || unmap.flags)
> >> +          if (unmap.argsz < minsz ||
> >> +              unmap.flags & ~VFIO_DMA_UNMAP_FLAG_GET_DIRTY_BITMAP)
> >>                    return -EINVAL;
> >>   
> >> -          ret = vfio_dma_do_unmap(iommu, &unmap);
> >> +          if (unmap.flags & VFIO_DMA_UNMAP_FLAG_GET_DIRTY_BITMAP) {
> >> +                  unsigned long pgshift;
> >> +
> >> +                  if (unmap.argsz < (minsz + sizeof(bitmap)))
> >> +                          return -EINVAL;
> >> +
> >> +                  if (copy_from_user(&bitmap,
> >> +                                     (void __user *)(arg + minsz),
> >> +                                     sizeof(bitmap)))
> >> +                          return -EFAULT;
> >> +
> >> +                  if (!access_ok((void __user *)bitmap.data, bitmap.size))
> >> +                          return -EINVAL;
> >> +
> >> +                  pgshift = __ffs(bitmap.pgsize);
> >> +                  ret = verify_bitmap_size(unmap.size >> pgshift,
> >> +                                           bitmap.size);
> >> +                  if (ret)
> >> +                          return ret;
> >> +          }
> >> +
> >> +          ret = vfio_dma_do_unmap(iommu, &unmap, &bitmap);
> >>            if (ret)
> >>                    return ret;
> >>   
> >> diff --git a/include/uapi/linux/vfio.h b/include/uapi/linux/vfio.h
> >> index 123de3bc2dce..0a0c7315ddd6 100644
> >> --- a/include/uapi/linux/vfio.h
> >> +++ b/include/uapi/linux/vfio.h
> >> @@ -1048,12 +1048,22 @@ struct vfio_bitmap {
> >>    * field.  No guarantee is made to the user that arbitrary unmaps of iova
> >>    * or size different from those used in the original mapping call will
> >>    * succeed.
> >> + * VFIO_DMA_UNMAP_FLAG_GET_DIRTY_BITMAP should be set to get dirty bitmap
> >> + * before unmapping IO virtual addresses. When this flag is set, user must
> >> + * provide data[] as structure vfio_bitmap. User must allocate memory to 
> >> get
> >> + * bitmap, zero the bitmap memory and must set size of allocated memory in
> >> + * vfio_bitmap.size field. A bit in bitmap represents one page of user 
> >> provided
> >> + * page size in 'pgsize', consecutively starting from iova offset. Bit set
> >> + * indicates page at that offset from iova is dirty. Bitmap of pages in 
> >> the
> >> + * range of unmapped size is returned in vfio_bitmap.data
> >>    */
> >>   struct vfio_iommu_type1_dma_unmap {
> >>    __u32   argsz;
> >>    __u32   flags;
> >> +#define VFIO_DMA_UNMAP_FLAG_GET_DIRTY_BITMAP (1 << 0)
> >>    __u64   iova;                           /* IO virtual address */
> >>    __u64   size;                           /* Size of mapping (bytes) */
> >> +  __u8    data[];
> >>   };
> >>   
> >>   #define VFIO_IOMMU_UNMAP_DMA _IO(VFIO_TYPE, VFIO_BASE + 14)  
> >   
>