Re: [PATCH V1 4/5] vfio: VA suspend interface

Alex Williamson
2021-01-12T22:49:38Z

On Mon, 11 Jan 2021 16:15:02 -0500
Steven Sistare <steven.sistare@oracle.com> wrote:

> On 1/8/2021 4:15 PM, Alex Williamson wrote:
> > On Tue, 5 Jan 2021 07:36:52 -0800
> > Steve Sistare <steven.sistare@oracle.com> wrote:
> >
> >> Add interfaces that allow the underlying memory object of an iova
> >> range to be mapped to a new host virtual address in the host process:
> >>
> >> - VFIO_DMA_UNMAP_FLAG_SUSPEND for VFIO_IOMMU_UNMAP_DMA
> >> - VFIO_DMA_MAP_FLAG_RESUME flag for VFIO_IOMMU_MAP_DMA
> >> - VFIO_SUSPEND extension for VFIO_CHECK_EXTENSION
> >
> > Suspend and Resume can imply many things other than what's done here.
> > Should these be something more akin to INVALIDATE_VADDR and
> > REPLACE_VADDR?
>
> Agreed. I suspected we would discuss the names. Some possibilities:
>
> INVALIDATE_VADDR REPLACE_VADDR
> INV_VADDR SET_VADDR
> CLEAR_VADDR SET_VADDR
> SUSPEND_VADDR RESUME_VADDR
>
> >> The suspend interface blocks vfio translation of host virtual
> >> addresses in a range, but DMA to already-mapped pages continues.
> >> The resume interface records the new base VA and resumes translation.
> >> See comments in uapi/linux/vfio.h for more details.
> >>
> >> This is a partial implementation. Blocking is added in the next patch.
> >>
> >> Signed-off-by: Steve Sistare <steven.sistare@oracle.com>
> >> ---
> >> drivers/vfio/vfio_iommu_type1.c | 47 +++++++++++++++++++++++++++++++++++------
> >> include/uapi/linux/vfio.h | 16 ++++++++++++++
> >> 2 files changed, 57 insertions(+), 6 deletions(-)
> >>
> >> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
> >> index 3dc501d..2c164a6 100644
> >> --- a/drivers/vfio/vfio_iommu_type1.c
> >> +++ b/drivers/vfio/vfio_iommu_type1.c
> >> @@ -92,6 +92,7 @@ struct vfio_dma {
> >> int prot; /* IOMMU_READ/WRITE */
> >> bool iommu_mapped;
> >> bool lock_cap; /* capable(CAP_IPC_LOCK) */
> >> + bool suspended;
> >
> > Is there a value we could use for vfio_dma.vaddr that would always be
> > considered invalid, ex. ULONG_MAX?
>
> Yes, that could replace the suspend flag. That, plus changing the language from suspend
> to invalidate, will probably yield equally understandable code. I'll try it.

Thinking about this further, if we defined a VFIO_IOMMU_TYPE1_INV_VADDR
as part of the uapi, could we implement this with only a single flag on
the DMA_MAP ioctl? For example the user would call DMA_MAP with a flag
to set the vaddr, first to the invalid valid, then to a new value. It's
always seemed a bit awkward to use DMA_UNMAP to invalidate the vaddr
when the mapping is not actually unmapped. That might lean towards an
UPDATE or REPLACE flag.

> > We'd need to decide if we want to
> > allow users to create mappings (mdev-only) using an initial invalid
> > vaddr.
>
> Maybe. Not sure yet.

If we used the above, it almost seems strange not to allow it, but at
the same time we don't really want to have different rules for
different devices types. An initially valid vaddr doesn't seem
unreasonable... though we don't test it until the vendor driver tries
to pin or rw pages w/o IOMMU backing.

> >> struct task_struct *task;
> >> struct rb_root pfn_list; /* Ex-user pinned pfn list */
> >> unsigned long *bitmap;
> >> @@ -1080,7 +1081,7 @@ static int vfio_dma_do_unmap(struct vfio_iommu *iommu,
> >> int ret = 0, retries = 0;
> >> unsigned long pgshift;
> >> dma_addr_t iova;
> >> - unsigned long size;
> >> + unsigned long size, consumed;
> >
> > This could be scoped into the branch below.
>
> OK.
>
> >> mutex_lock(&iommu->lock);
> >>
> >> @@ -1169,6 +1170,21 @@ static int vfio_dma_do_unmap(struct vfio_iommu *iommu,
> >> if (dma->task->mm != current->mm)
> >> break;
> >>
> >> + if (unmap->flags & VFIO_DMA_UNMAP_FLAG_SUSPEND) {
> >> + if (dma->suspended) {
> >> + ret = -EINVAL;
> >> + goto unlock;
> >> + }
> >
> > This leaves us in a state where we marked some entries but not others.
> > We should either unwind or... what's the actual harm in skipping these?
>
> We could skip them with no ill effect. However, it likely means the app is confused
> and potentially broken, and it would be courteous to inform them so. I found such bugs
> in qemu as I was developing this feature.
>
> IMO unwinding does not help the app, and adds unnecessary code. It can still leave some
> ranges suspended and some not. The safest recovery is for the app to exit, and tell the
> developer to fix the redundant suspend call.

That sounds like an entirely practical rationalization, but our
standard practice is to maintain a consistent state. If an ioctl fails
is should effectively be as if the ioctl was never called, where
possible. Userspace can be broken, and potentially so broken that their
best choice is to abort, but we should maintain consistent, predictable
behavior.

> >> + dma->suspended = true;
> >> + consumed = dma->iova + dma->size - iova;
> >> + if (consumed >= size)
> >> + break;
> >> + iova += consumed;
> >> + size -= consumed;
> >> + unmapped += dma->size;
> >> + continue;
> >> + }
> >
> > This short-cuts the dirty bitmap flag, so we need to decide if it's
> > legal to call them together or we need to prevent it... Oh, I see
> > you've excluded them earlier below.
> >
> >> +
> >> if (!RB_EMPTY_ROOT(&dma->pfn_list)) {
> >> struct vfio_iommu_type1_dma_unmap nb_unmap;
> >>
> >> @@ -1307,6 +1323,7 @@ static bool vfio_iommu_iova_dma_valid(struct vfio_iommu *iommu,
> >> static int vfio_dma_do_map(struct vfio_iommu *iommu,
> >> struct vfio_iommu_type1_dma_map *map)
> >> {
> >> + bool resume = map->flags & VFIO_DMA_MAP_FLAG_RESUME;
> >> dma_addr_t iova = map->iova;
> >> unsigned long vaddr = map->vaddr;
> >> size_t size = map->size;
> >> @@ -1324,13 +1341,16 @@ static int vfio_dma_do_map(struct vfio_iommu *iommu,
> >> if (map->flags & VFIO_DMA_MAP_FLAG_READ)
> >> prot |= IOMMU_READ;
> >>
> >> + if ((prot && resume) || (!prot && !resume))
> >> + return -EINVAL;
> >> +
> >> mutex_lock(&iommu->lock);
> >>
> >> pgsize = (size_t)1 << __ffs(iommu->pgsize_bitmap);
> >>
> >> WARN_ON((pgsize - 1) & PAGE_MASK);
> >>
> >> - if (!prot || !size || (size | iova | vaddr) & (pgsize - 1)) {
> >> + if (!size || (size | iova | vaddr) & (pgsize - 1)) {
> >> ret = -EINVAL;
> >> goto out_unlock;
> >> }
> >> @@ -1341,7 +1361,19 @@ static int vfio_dma_do_map(struct vfio_iommu *iommu,
> >> goto out_unlock;
> >> }
> >>
> >> - if (vfio_find_dma(iommu, iova, size)) {
> >> + dma = vfio_find_dma(iommu, iova, size);
> >> + if (resume) {
> >> + if (!dma) {
> >> + ret = -ENOENT;
> >> + } else if (!dma->suspended || dma->iova != iova ||
> >> + dma->size != size) {
> >
> > Why is it necessary that the vfio_dma be suspended before being
> > resumed? Couldn't a user simply use this to change the vaddr? Does
> > that promote abusive use?
>
> This would almost always be incorrect. If the vaddr changes, then the old vaddr was already
> invalidated, and there is a window where it is not OK for kernel code to use the old vaddr.
> This could only be safe if the memory object is mapped at both the old vaddr and the new
> vaddr concurrently, which is an unlikely use case.

Ok, it's not like the use can't make it instantaneously invalid and then
replace it.

> >> + ret = -EINVAL;
> >> + } else {
> >> + dma->vaddr = vaddr;
> >
> > Seems like there's a huge opportunity for a user to create coherency
> > issues here... it's their data though I guess.
>
> Yes. That's what the language in the uapi about mapping the same memory object is about.
>
> >> + dma->suspended = false;
> >> + }
> >> + goto out_unlock;
> >> + } else if (dma) {
> >> ret = -EEXIST;
> >> goto out_unlock;
> >> }
> >> @@ -2532,6 +2564,7 @@ static int vfio_iommu_type1_check_extension(struct vfio_iommu *iommu,
> >> case VFIO_TYPE1_IOMMU:
> >> case VFIO_TYPE1v2_IOMMU:
> >> case VFIO_TYPE1_NESTING_IOMMU:
> >> + case VFIO_SUSPEND:
> >> return 1;
> >> case VFIO_DMA_CC_IOMMU:
> >> if (!iommu)
> >> @@ -2686,7 +2719,8 @@ static int vfio_iommu_type1_map_dma(struct vfio_iommu *iommu,
> >> {
> >> struct vfio_iommu_type1_dma_map map;
> >> unsigned long minsz;
> >> - uint32_t mask = VFIO_DMA_MAP_FLAG_READ | VFIO_DMA_MAP_FLAG_WRITE;
> >> + uint32_t mask = VFIO_DMA_MAP_FLAG_READ | VFIO_DMA_MAP_FLAG_WRITE |
> >> + VFIO_DMA_MAP_FLAG_RESUME;
> >>
> >> minsz = offsetofend(struct vfio_iommu_type1_dma_map, size);
> >>
> >> @@ -2704,6 +2738,8 @@ static int vfio_iommu_type1_unmap_dma(struct vfio_iommu *iommu,
> >> {
> >> struct vfio_iommu_type1_dma_unmap unmap;
> >> struct vfio_bitmap bitmap = { 0 };
> >> + uint32_t mask = VFIO_DMA_UNMAP_FLAG_GET_DIRTY_BITMAP |
> >> + VFIO_DMA_UNMAP_FLAG_SUSPEND;
> >> unsigned long minsz;
> >> int ret;
> >>
> >> @@ -2712,8 +2748,7 @@ static int vfio_iommu_type1_unmap_dma(struct vfio_iommu *iommu,
> >> if (copy_from_user(&unmap, (void __user *)arg, minsz))
> >> return -EFAULT;
> >>
> >> - if (unmap.argsz < minsz ||
> >> - unmap.flags & ~VFIO_DMA_UNMAP_FLAG_GET_DIRTY_BITMAP)
> >> + if (unmap.argsz < minsz || unmap.flags & ~mask || unmap.flags == mask)
> >
> > Maybe a short comment here to note that dirty-bimap and
> > suspend/invalidate are mutually exclusive. Probably should be
> > mentioned in the uapi too.
>
> Will do, for both.
>
> >> return -EINVAL;
> >>
> >> if (unmap.flags & VFIO_DMA_UNMAP_FLAG_GET_DIRTY_BITMAP) {
> >> diff --git a/include/uapi/linux/vfio.h b/include/uapi/linux/vfio.h
> >> index 896e527..fcf7b56 100644
> >> --- a/include/uapi/linux/vfio.h
> >> +++ b/include/uapi/linux/vfio.h
> >> @@ -46,6 +46,9 @@
> >> */
> >> #define VFIO_NOIOMMU_IOMMU 8
> >>
> >> +/* Supports VFIO DMA suspend and resume */
> >> +#define VFIO_SUSPEND 9
> >> +
> >> /*
> >> * The IOCTL interface is designed for extensibility by embedding the
> >> * structure length (argsz) and flags into structures passed between
> >> @@ -1046,12 +1049,19 @@ struct vfio_iommu_type1_info_cap_migration {
> >> *
> >> * Map process virtual addresses to IO virtual addresses using the
> >> * provided struct vfio_dma_map. Caller sets argsz. READ &/ WRITE required.
> >> + *
> >> + * If flags & VFIO_DMA_MAP_FLAG_RESUME, record the new base vaddr for iova, and
> >> + * resume translation of host virtual addresses in the iova range. The new
> >> + * vaddr must point to the same memory object as the old vaddr, but this is not
> >> + * verified.
> >
> > It's hard to use "must" terminology here if we're not going to check.
> > Maybe the phrasing should be something more along the lines of "should
> > point to the same memory object or the user risks coherency issues
> > within their virtual address space".
>
> I used "must" because it is always incorrect if the object is not the same. How about:
> The new vaddr must point to the same memory object as the old vaddr, but this is not
> verified. Violation of this constraint may result in memory corruption within the
> host process and/or guest.

Since the "must" is not relative to the API but to the resulting
behavior, perhaps something like:

In order to maintain memory consistency within the user application,
the updated vaddr must address the same memory object as originally
mapped, failure to do so will result in user memory corruption and/or
device misbehavior.

Thanks,
Alex

> >> iova and size must match those in the original MAP_DMA call.
> >> + * Protection is not changed, and the READ & WRITE flags must be 0.
> >
> > This doesn't mention that the entry must be previously
> > suspended/invalidated (if we choose to keep those semantics). Thanks,
>
> Will add, thanks.
>
> - Steve
> >> */
> >> struct vfio_iommu_type1_dma_map {
> >> __u32 argsz;
> >> __u32 flags;
> >> #define VFIO_DMA_MAP_FLAG_READ (1 << 0) /* readable from device */
> >> #define VFIO_DMA_MAP_FLAG_WRITE (1 << 1) /* writable from device */
> >> +#define VFIO_DMA_MAP_FLAG_RESUME (1 << 2)
> >> __u64 vaddr; /* Process virtual address */
> >> __u64 iova; /* IO virtual address */
> >> __u64 size; /* Size of mapping (bytes) */
> >> @@ -1084,11 +1094,17 @@ struct vfio_bitmap {
> >> * indicates that the page at that offset from iova is dirty. A Bitmap of the
> >> * pages in the range of unmapped size is returned in the user-provided
> >> * vfio_bitmap.data.
> >> + *
> >> + * If flags & VFIO_DMA_UNMAP_FLAG_SUSPEND, do not unmap, but suspend vfio
> >> + * translation of host virtual addresses in the iova range. During suspension,
> >> + * kernel threads that attempt to translate will block. DMA to already-mapped
> >> + * pages continues.
> >> */
> >> struct vfio_iommu_type1_dma_unmap {
> >> __u32 argsz;
> >> __u32 flags;
> >> #define VFIO_DMA_UNMAP_FLAG_GET_DIRTY_BITMAP (1 << 0)
> >> +#define VFIO_DMA_UNMAP_FLAG_SUSPEND (1 << 1)
> >> __u64 iova; /* IO virtual address */
> >> __u64 size; /* Size of mapping (bytes) */
> >> __u8 data[];
> >
>