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

Kirti Wankhede
2020-05-14T05:32:48Z

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?
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.

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;
}

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;
}
Then current implementation should work.

Thanks,
Kirti

> 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)
>