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

Alex Williamson
2021-01-13T04:12:09Z

On Tue, 12 Jan 2021 15:47:56 -0700
Alex Williamson <alex.williamson@redhat.com> wrote:

> 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.

I realized you really want to make use of the DMA_UNMAP ioctl in order
to use ranges, maybe we can make the mental model more coherent with an
unmap flag like VFIO_DMA_UNMAP_FLAG_VADDR_ONLY, ie. we're only asking
to unmap the vaddr. The DMA_MAP side might take a similar VADDR_ONLY
flag to reset the vaddr. That also retains your desired semantics that
we can't "resume" a vaddr that wasn't previously "suspended", we can't
map a vaddr that wasn't previously unmapped.

For the unmap-all problem, userspace already needs to work around this,
see for instance QEMU:

1b296c3def4b vfio: Don't issue full 2^64 unmap

So I wonder really how critical it is and whether it really would be
sufficient for userspace to track a high water mark for mappings.
Otherwise, I think I'm leaning towards a DMA_UNMAP flag like
VFIO_DMA_UNMAP_FLAG_UNMAP_ALL that would disregard the iova and size
fields to apply to all mappings. Designating special values for iova or
size trigger extended behavior feels a bit hackish. Thanks,

Alex

> > > 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[];
> > >
> >
>