Re: [PATCH V1 2/5] vfio: option to unmap all

Alex Williamson
2021-01-13T19:43:17Z

On Mon, 11 Jan 2021 16:09:18 -0500
Steven Sistare <steven.sistare@oracle.com> wrote:

> On 1/8/2021 2:35 PM, Alex Williamson wrote:
> > Hi Steve,
> >
> > On Tue, 5 Jan 2021 07:36:50 -0800
> > Steve Sistare <steven.sistare@oracle.com> wrote:
> >
> >> For VFIO_IOMMU_UNMAP_DMA, delete all mappings if iova=0 and size=0.
> >
> > Only the latter is invalid, iova=0 is not special, so does it make
> > sense to use this combination to invoke something special? It seems
> > like it opens the door that any size less than the minimum mapping
> > granularity means something special.
> >
> > Why not use a flag to trigger an unmap-all?
>
> Hi Alex, that would be fine.
>
> > Does userspace have any means to know this is supported other than to
> > test it before creating any mappings?
>
> Not currently. We could overload VFIO_SUSPEND, or define a new extension code.

Either an extension or a capability on the IOMMU_INFO return data.
If I interpret our trend on which to use, an extension seems
appropriate here as were only indicating support for a feature with no
additional data to return.

> > What's the intended interaction with retrieving the dirty bitmap during
> > an unmap-all?
>
> Undefined and broken if there are gaps between segments :( Good catch, thanks.
> I will disallow the combination of unmap-all and get-dirty-bitmap.
>
> >> Signed-off-by: Steve Sistare <steven.sistare@oracle.com>
> >> ---
> >> drivers/vfio/vfio_iommu_type1.c | 11 ++++++++---
> >> include/uapi/linux/vfio.h | 3 ++-
> >> 2 files changed, 10 insertions(+), 4 deletions(-)
> >>
> >> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
> >> index 02228d0..3dc501d 100644
> >> --- a/drivers/vfio/vfio_iommu_type1.c
> >> +++ b/drivers/vfio/vfio_iommu_type1.c
> >> @@ -1079,6 +1079,8 @@ static int vfio_dma_do_unmap(struct vfio_iommu *iommu,
> >> size_t unmapped = 0, pgsize;
> >> int ret = 0, retries = 0;
> >> unsigned long pgshift;
> >> + dma_addr_t iova;
> >> + unsigned long size;
> >>
> >> mutex_lock(&iommu->lock);
> >>
> >> @@ -1090,7 +1092,7 @@ static int vfio_dma_do_unmap(struct vfio_iommu *iommu,
> >> goto unlock;
> >> }
> >>
> >> - if (!unmap->size || unmap->size & (pgsize - 1)) {
> >> + if ((!unmap->size && unmap->iova) || unmap->size & (pgsize - 1)) {
> >> ret = -EINVAL;
> >> goto unlock;
> >> }
> >> @@ -1154,8 +1156,11 @@ static int vfio_dma_do_unmap(struct vfio_iommu *iommu,
> >
> > It looks like the code just above this would have an issue if there are
> > dma mappings at iova=0.
>
> Are you referring to this code?
>
> if (iommu->v2) {
> dma = vfio_find_dma(iommu, unmap->iova, 1);
> if (dma && dma->iova != unmap->iova) {
> ret = -EINVAL;
>
> Both unmap->iova and dma->iova would be 0, so I don't see the problem.

Yeah, I think I was mistaken. Thanks,

Alex

> >> }
> >> }
> >>
> >> - while ((dma = vfio_find_dma(iommu, unmap->iova, unmap->size))) {
> >> - if (!iommu->v2 && unmap->iova > dma->iova)
> >> + iova = unmap->iova;
> >> + size = unmap->size ? unmap->size : SIZE_MAX;
> >
> > AFAICT the only difference of this versus the user calling the unmap
> > with iova=0 size=SIZE_MAX is that SIZE_MAX will throw an -EINVAL due to
> > page size alignment. If we assume there are no IOMMUs with 1 byte page
> > size, the special combination could instead be {0, SIZE_MAX}.
>
> Fine, but we would still need to document it specifically so the user knows that
> the unaligned SIZE_MAX does not return EINVAL.
>
> > Or the
> > caller could just track a high water mark for their mappings and use
> > the interface that exists. Thanks,
>
> I am trying to avoid the need to modify existing code, for legacy qemu live update.
> Either a new flag or {0, SIZE_MAX} is suitable. Which do you prefer?
>
> - Steve
>
> >> +
> >> + while ((dma = vfio_find_dma(iommu, iova, size))) {
> >> + if (!iommu->v2 && iova > dma->iova)
> >> break;
> >> /*
> >> * Task with same address space who mapped this iova range is
> >> diff --git a/include/uapi/linux/vfio.h b/include/uapi/linux/vfio.h
> >> index 9204705..896e527 100644
> >> --- a/include/uapi/linux/vfio.h
> >> +++ b/include/uapi/linux/vfio.h
> >> @@ -1073,7 +1073,8 @@ struct vfio_bitmap {
> >> * Caller sets argsz. The actual unmapped size is returned in the size
> >> * 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.
> >> + * succeed. If iova=0 and size=0, all addresses are unmapped.
> >> + *
> >> * VFIO_DMA_UNMAP_FLAG_GET_DIRTY_BITMAP should be set to get the dirty bitmap
> >> * before unmapping IO virtual addresses. When this flag is set, the user must
> >> * provide a struct vfio_bitmap in data[]. User must provide zero-allocated
> >
>