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

[Steven Sistare <steven.sistare@oracle.com>]

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.

> 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.

>>  	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.

>> +			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.

>> +			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.

>>  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[];
>