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

Steven Sistare
2021-01-11T21:12:23Z

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.

> 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.

>> }
>> }
>>
>> - 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
>