Re: [PATCH Kernel v18 5/7] vfio iommu: Update UNMAP_DMA ioctl to get dirty bitmap before unmap

[Kirti Wankhede]

On 5/7/2020 3:55 AM, Alex Williamson wrote:
> On Mon, 4 May 2020 21:28:57 +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
> > 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 | 84 +++++++++++++++++++++++++++++++++++++++--
> >   include/uapi/linux/vfio.h       | 10 +++++
> >   2 files changed, 90 insertions(+), 4 deletions(-)
> >
> > diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
> > index 01dcb417836f..8b27faf1ec38 100644
> > --- a/drivers/vfio/vfio_iommu_type1.c
> > +++ b/drivers/vfio/vfio_iommu_type1.c
> > @@ -983,12 +983,14 @@ 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;
> > +       unsigned long *final_bitmap = NULL, *temp_bitmap = NULL;
> >   
> >   	mask = ((uint64_t)1 << __ffs(vfio_pgsize_bitmap(iommu))) - 1;
> >   
> > @@ -1041,6 +1043,7 @@ 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;
> > @@ -1048,6 +1051,22 @@ static int vfio_dma_do_unmap(struct vfio_iommu *iommu,
> >                 }
> >         }
> >   
> > +	if ((unmap->flags & VFIO_DMA_UNMAP_FLAG_GET_DIRTY_BITMAP) &&
> > +            iommu->dirty_page_tracking) {
>
> Why do we even accept VFIO_DMA_UNMAP_FLAG_GET_DIRTY_BITMAP when not
> dirty page tracking rather than returning -EINVAL?  It would simplify
> things here to reject it at the ioctl and silently ignoring a flag is
> rarely if ever the right approach.
>
> > +               final_bitmap = kvzalloc(bitmap->size, GFP_KERNEL);
> > +               if (!final_bitmap) {
> > +                       ret = -ENOMEM;
> > +                       goto unlock;
> > +               }
> > +
> > +               temp_bitmap = kvzalloc(bitmap->size, GFP_KERNEL);
> > +               if (!temp_bitmap) {
> > +                       ret = -ENOMEM;
> > +                       kfree(final_bitmap);
> > +                       goto unlock;
> > +               }
>
> YIKES!  So the user can instantly trigger the kernel to internally
> allocate 2 x 256MB, regardless of how much they can actually map.

That is worst case senario. I don't think ideally that will ever hit. 
More comment below regarding this.

> > +       }
> > +
> >         while ((dma = vfio_find_dma(iommu, unmap->iova, unmap->size))) {
> >                 if (!iommu->v2 && unmap->iova > dma->iova)
> >                         break;
> > @@ -1058,6 +1077,24 @@ 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) &&
> > +                    iommu->dirty_page_tracking) {
> > +                       unsigned long pgshift = __ffs(bitmap->pgsize);
> > +                       unsigned int npages = dma->size >> pgshift;
> > +                       unsigned int shift;
> > +
> > +                       vfio_iova_dirty_bitmap(iommu, dma->iova, dma->size,
> > +                                       bitmap->pgsize, (u64 *)temp_bitmap);
>
> vfio_iova_dirty_bitmap() takes a __user bitmap, we're doing
> copy_to_user() on a kernel allocated buffer???

Actually, there is no need to call vfio_iova_dirty_bitmap(), dma pointer 
is known here and since its getting unmapped, there is no need to 
repopulate bitmap. Removing vfio_iova_dirty_bitmap() and changing it as 
below:

if (unmap->flags & VFIO_DMA_UNMAP_FLAG_GET_DIRTY_BITMAP) {
    unsigned long pgshift = __ffs(bitmap->pgsize);
    unsigned int npages = dma->size >> pgshift;
    unsigned int bitmap_size = DIRTY_BITMAP_BYTES(npages);
    unsigned int shift = (dma->iova - unmap->iova) >>
                                            pgshift;
    /*
     * mark all pages dirty if all pages are pinned and
     * mapped.
     */
    if (dma->iommu_mapped)
        bitmap_set(temp_bitmap, 0, npages);
    else
        memcpy(temp_bitmap, dma->bitmap, bitmap_size);

    if (shift)
        bitmap_shift_left(temp_bitmap, temp_bitmap,
                          shift, npages);
    bitmap_or(final_bitmap, final_bitmap, temp_bitmap,
              shift + npages);
    memset(temp_bitmap, 0, bitmap->size);
}

> > +
> > +                       shift = (dma->iova - unmap->iova) >> pgshift;
> > +                       if (shift)
> > +                               bitmap_shift_left(temp_bitmap, temp_bitmap,
> > +                                                 shift, npages);
> > +                       bitmap_or(final_bitmap, final_bitmap, temp_bitmap,
> > +                                 shift + npages);
> > +                       memset(temp_bitmap, 0, bitmap->size);
> > +               }
>
> It seems like if the per vfio_dma dirty bitmap was oversized by a long
> that we could shift it in place, then we'd only need one working bitmap
> buffer and we could size that to fit the vfio_dma (or the largest
> vfio_dma if we don't want to free and re-alloc for each vfio_dma).
> We'd need to do more copy_to/from_user()s, but we'd also avoid copying
> between sparse mappings (user zero'd bitmap required) and we'd have a
> far more reasonable memory usage.  Thanks,

I thought about it, but couldn't optimize to use one bitmap buffer.
This case will only hit during migration with vIOMMU enabled.
Can we keep these 2 bitmap buffers for now and optimize it later?

Thanks,
Kirti