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

Alex Williamson
2020-05-14T13:22:27Z

On Thu, 14 May 2020 11:02:33 +0530
Kirti Wankhede <kwankhede@nvidia.com> wrote:

> On 5/14/2020 10:37 AM, Alex Williamson wrote:
> > 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.
>
> Unmap range should include adjacent mapped ranges, right?

Not required.

> In your example, if user asks for range 0-16k but mapping at 0 wasn't
> done, then this unmap would fail before even reaching control here.

Nope, that's supported.

> There is a check which makes sure that mapping for start of range exist:
>
> dma = vfio_find_dma(iommu, unmap->iova, 1);
> if (dma && dma->iova != unmap->iova) {
> ret = -EINVAL;
> goto unlock;
> }

if (dma && ...

> There is a check which makes sure that mapping for last address of range
> exist:
> dma = vfio_find_dma(iommu, unmap->iova + unmap->size - 1, 0);
> if (dma && dma->iova + dma->size != unmap->iova + unmap->size) {
> ret = -EINVAL;
> goto unlock;
> }

if (dma && ...

These are the tests that require an unmap request not _bisect_ a
mapping, both tests are preceded by a check of if we found a mapping at
that address, not a requirement that there be a mapping at that
address, nor a requirement that mappings are consecutive across the
unmap range. A user can absolutely call unmap with iova = 0, size =
S64_MAX to clear a massive chunk of the address space.

> Then current implementation should work.

Even if I change my example that the user requested an unmap of 4K-16K
with 4K mappings at 4K and 12K, the bitmap returned would have bits
0 & 1 set rather than bits 0 & 2, which is still incorrect. Thanks,

Alex

> > 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)
> >
>