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

Steven Sistare
2021-01-13T18:02:41Z

On 1/12/2021 5:47 PM, Alex Williamson 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 like your VADDR_ONLY suggestion in your next email better.

>>> 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.

Agreed, I take it back. Or unwind it?
What's that Big Chill quote: ever gone a week without a rationalization?

I'll unwind and return EINVAL.

>>>> + 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.

Sounds good.

- Steve

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