Re: [PATCH Kernel v19 6/8] vfio iommu: Update UNMAP_DMA ioctl to get dirty bitmap before unmap

Alex Williamson
2020-05-14T05:08:00Z

On Thu, 14 May 2020 01:34:37 +0530
Kirti Wankhede <kwankhede@nvidia.com> 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
> 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 <kwankhede@nvidia.com>
> Reviewed-by: Neo Jia <cjia@nvidia.com>
> ---
> drivers/vfio/vfio_iommu_type1.c | 102 +++++++++++++++++++++++++++++++++++-----
> include/uapi/linux/vfio.h | 10 ++++
> 2 files changed, 99 insertions(+), 13 deletions(-)
>
> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
> index 469b09185b83..4358be26ff80 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;
>
> @@ -979,23 +983,25 @@ 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;
> - int ret = 0, retries = 0;
> + size_t unmapped = 0, pgsize;
> + int ret = 0, retries = 0, cnt = 0;
> + unsigned long pgshift, shift = 0, leftover;
>
> 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;
> }
> @@ -1006,9 +1012,15 @@ 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
> @@ -1046,6 +1058,7 @@ 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;
> @@ -1063,6 +1076,39 @@ 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)) {
> + unsigned int nbits = dma->size >> pgshift;
> + int curr_lcnt = nbits / BITS_PER_LONG;
> +
> + /*
> + * mark all pages dirty if all pages are pinned and
> + * mapped.
> + */
> + if (dma->iommu_mapped)
> + bitmap_set(dma->bitmap, 0, nbits);
> +
> + if (shift) {
> + bitmap_shift_left(dma->bitmap, dma->bitmap,
> + shift, nbits + shift);
> + bitmap_or(dma->bitmap, dma->bitmap, &leftover,
> + shift);
> + nbits += shift;
> + curr_lcnt = nbits / BITS_PER_LONG;
> + }
> +
> + if (copy_to_user((void __user *)bitmap->data + cnt,
> + dma->bitmap, curr_lcnt * sizeof(u64))) {
> + ret = -EFAULT;
> + break;
> + }
> +
> + shift = nbits % BITS_PER_LONG;
> + if (shift)
> + leftover = *(u64 *)(dma->bitmap + curr_lcnt);
> + cnt += curr_lcnt;
> + }

I don't think this works. Let's say for example we have separate
single page mappings at 4K and 12K (both dirty) and the user asked to
unmap the range 0 - 16K. We find the mapping at 4K, shift is zero, cnt
is zero, so we copy the bitmap with the zero bit set to the user
buffer. We're already wrong because we've just indicated the page at
zero is dirty and there isn't a page at zero. shift now becomes 1 and
leftover is a bitmap with bit zero set.

We move on to the next page @12K. We shift this bitmap by 1. We OR in
our leftover and again copy out to the user buffer. We end up with a
user bitmap with bits zero and one set, when we should have had bits 1
and 3 set, we're essentially coalescing the mappings.

As I see it, shift needs to be calculated as the offset from the start
of the user requested unmap buffer and I think an easier approach to
handle the leftover bits preceding the shift is to copy it back out of
the user buffer.

For example, shift should be:

((dma->iova - unmap->iova) >> pgshift) % BITS_PER_LONG

This would give us a shift of 1 and 3 respectively for our mappings,
which is correct.

Since our shifts are non-zero, we then need to collect the preceding
leftovers, which is always going to be:

copy_from_user(&leftover, bitmap->data +
((dma->iova - unmap->iova) >> pgshift) / BITS_PER_LONG,
sizeof(leftover));

I don't think the curr_lcnt calculation for the copy-out is correct
either, mappings are not required to be a multiple of BITS_PER_LONG
pages, so we're truncating the size.

So we have:

bit_offset = (dma->iova - unmap->iova) >> pgshift;
copy_offset = bit_offset / BITS_PER_LONG;
shift = bit_offset % BITS_PER_LONG;

if (shift) {
bitmap_shift_left(dma->bitmap, dma->bitmap, shift, nbits + shift);
if (copy_from_user(&leftover, bitmap->data + copy_offset, sizeof(leftover))) {
ret = -EFAULT;
break;
}
bitmap_or(dma->bitmap, dma->bitmap, &leftover, shift);
}

if (copy_to_user(bitmap->data + copy_offset, dma->bitmap,
roundup(nbits + shift, BITS_PER_LONG)/BITS_PER_BYTE)) {
ret = -EFAULT;
break;
}

Also this all needs to come after the below check of the pfn_list and
call to the blocking notifier or else we're just wasting time because
we'll need to do it all again anyway.

> +
> if (!RB_EMPTY_ROOT(&dma->pfn_list)) {
> struct vfio_iommu_type1_dma_unmap nb_unmap;
>
> @@ -1093,6 +1139,13 @@ static int vfio_dma_do_unmap(struct vfio_iommu *iommu,
> vfio_remove_dma(iommu, dma);
> }
>
> + if (!ret && (unmap->flags & VFIO_DMA_UNMAP_FLAG_GET_DIRTY_BITMAP) &&
> + shift) {
> + if (copy_to_user((void __user *)bitmap->data + cnt, &leftover,
> + sizeof(leftover)))
> + ret = -EFAULT;
> + }

This is unnecessary with the algorithm I propose.

> +
> unlock:
> mutex_unlock(&iommu->lock);
>
> @@ -2426,17 +2479,40 @@ 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 5f359c63f5ef..e3cbf8b78623 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 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

This needs to specify a user zero'd bitmap if we're only going to fill
it sparsely. Thanks,

Alex

> */
> 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)