Re: [PATCH v17 Kernel 5/7] vfio iommu: Update UNMAP_DMA ioctl to get dirty bitmap before unmap

[Alex Williamson <alex.williamson@redhat.com>]

On Wed, 1 Apr 2020 00:16:13 +0530
Kirti Wankhede <kwankhede@nvidia.com> wrote:

> On 3/31/2020 3:04 AM, Alex Williamson wrote:
> > On Mon, 30 Mar 2020 22:20:42 +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 | 55 ++++++++++++++++++++++++++++++++++++++---
> >>   include/uapi/linux/vfio.h       | 10 ++++++++
> >>   2 files changed, 61 insertions(+), 4 deletions(-)
> >>
> >> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
> >> index 5efebc2b60e1..266550bd7307 100644
> >> --- a/drivers/vfio/vfio_iommu_type1.c
> >> +++ b/drivers/vfio/vfio_iommu_type1.c
> >> @@ -983,7 +983,8 @@ 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;
> >> @@ -1034,6 +1035,10 @@ static int vfio_dma_do_unmap(struct vfio_iommu *iommu,
> >>   	 * will be returned if these conditions are not met.  The v2 interface
> >>   	 * will only return success and a size of zero if there were no
> >>   	 * mappings within the range.
> >> +	 *
> >> +	 * When VFIO_DMA_UNMAP_FLAG_GET_DIRTY_BITMAP flag is set, unmap request
> >> +	 * must be for single mapping. Multiple mappings with this flag set is
> >> +	 * not supported.
> >>   	 */
> >>   	if (iommu->v2) {
> >>   		dma = vfio_find_dma(iommu, unmap->iova, 1);
> >> @@ -1041,6 +1046,14 @@ static int vfio_dma_do_unmap(struct vfio_iommu *iommu,
> >>   			ret = -EINVAL;
> >>   			goto unlock;
> >>   		}
> >> +
> >> +		if ((unmap->flags & VFIO_DMA_UNMAP_FLAG_GET_DIRTY_BITMAP) &&
> >> +		    dma &&
> >> +		    (dma->iova != unmap->iova || dma->size != unmap->size)) {  
> > 
> > 
> > I think your intention was to return error if the user asked for the
> > dirty bitmap and the requested unmap range doesn't exactly match the
> > vfio_dma.  Not finding a vfio_dma should therefore also be an error.
> > For example, if we had a single mapping at {0x1000-0x1fff} and the user
> > unmapped with dirty bitmap {0x0-0x2fff}, that should return an error,
> > but it's not caught by the above because there is no vfio_dma @0x0.
> > Therefore I think you want:
> > 
> > ((unmap->flags & VFIO_DMA_UNMAP_FLAG_GET_DIRTY_BITMAP) &&
> >   (!dma || dma->iova != unmap->iova || dma->size != unmap->size))
> > 
> > Right?  Thanks,
> >   
> 
> 
> Yes, updating check.
> 
> Is !dma here also error case when VFIO_DMA_UNMAP_FLAG_GET_DIRTY_BITMAP 
> flag is not set?

No it's not.

> DMA_UNMAP ioctl returns how much was unmapped, from user space 
> perspective this would be from start of range (unmap->iova), right?

It's the actual amount of memory that was unmapped within the provided
range, without getting the dirty bitmap the range doesn't need to start
or end at a vfio_dma, it just cannot bisect a vfio_dma.  Thanks,

Alex