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

Alex Williamson
2021-01-13T18:35:50Z

On Wed, 13 Jan 2021 13:02:04 -0500
Steven Sistare <steven.sistare@oracle.com> wrote:

> On 1/12/2021 11:10 PM, Alex Williamson wrote:
> > 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.
>
> I like this.
>
> > 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
>
> Yes, I saw that. And one cannot split the range in two at some arbitrary
> mmu-page-size boundary, least you split a mapping, which V2 does not allow.

Yes, we noted this shortfall in its design, but we're also not
currently in danger of using anything in the upper half of the 64-bit
address space. In fact vfio_listener_skipped_section() uses bit 63 to
filter out PCI BAR sizing operations that the QEMU PCI core erroneously
sends out to MemoryListeners.

> > 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.
>
> If the high water mark is 2^64, then you also need a low water mark to
> enable a single call, else one call cannot span the range. And low water
> better not be 0. And I would rather not add code to compute either one :)

The high water marker would track the highest iova+size < 2^64. If an
unmap occurs that covers that high water marker, the marker is reset to
the start of the unmap. If an unmap of the full 64-bit address space
is requested, userspace performs 2 unmaps, one for the area below the
marker, one for the area above and resets the marker to zero. The
mechanics seem pretty simple, we don't need to cover the entire used
address space in a single unmap, we only need to track a dividing line
to split the address space. Anyway, it's been a nagging oversight in
the original design and I'm not opposed to solving it with a flag to
unburden userspace from such things. Thanks,

Alex

> > 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,
>
> The flag sounds best to me.
>
> - Steve
>
> >>>> 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[];
> >>>>
> >>>
> >>
> >
>