Re: [PATCH Kernel v18 5/7] vfio iommu: Update UNMAP_DMA ioctl to get dirty bitmap before unmap

[Kirti Wankhede <kwankhede@nvidia.com>]

On 5/7/2020 3:55 AM, Alex Williamson wrote:
> On Mon, 4 May 2020 21:28:57 +0530
> Kirti Wankhede <kwankhede@nvidia.com> wrote:
> 
>> DMA mapped pages, including those pinned by mdev vendor drivers, might
>> get unpinned and unmapped while migration is active and device is still
>> running. For example, in pre-copy phase while guest driver could access
>> those pages, host device or vendor driver can dirty these mapped pages.
>> Such pages should be marked dirty so as to maintain memory consistency
>> for a user making use of dirty page tracking.
>>
>> To get bitmap during unmap, user should allocate memory for bitmap, set
>> size of allocated memory, set page size to be considered for bitmap and
>> set flag VFIO_DMA_UNMAP_FLAG_GET_DIRTY_BITMAP.
>>
>> Signed-off-by: Kirti Wankhede <kwankhede@nvidia.com>
>> Reviewed-by: Neo Jia <cjia@nvidia.com>
>> ---
>>   drivers/vfio/vfio_iommu_type1.c | 84 +++++++++++++++++++++++++++++++++++++++--
>>   include/uapi/linux/vfio.h       | 10 +++++
>>   2 files changed, 90 insertions(+), 4 deletions(-)
>>
>> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
>> index 01dcb417836f..8b27faf1ec38 100644
>> --- a/drivers/vfio/vfio_iommu_type1.c
>> +++ b/drivers/vfio/vfio_iommu_type1.c
>> @@ -983,12 +983,14 @@ static int verify_bitmap_size(uint64_t npages, uint64_t bitmap_size)
>>   }
>>   
>>   static int vfio_dma_do_unmap(struct vfio_iommu *iommu,
>> -			     struct vfio_iommu_type1_dma_unmap *unmap)
>> +			     struct vfio_iommu_type1_dma_unmap *unmap,
>> +			     struct vfio_bitmap *bitmap)
>>   {
>>   	uint64_t mask;
>>   	struct vfio_dma *dma, *dma_last = NULL;
>>   	size_t unmapped = 0;
>>   	int ret = 0, retries = 0;
>> +	unsigned long *final_bitmap = NULL, *temp_bitmap = NULL;
>>   
>>   	mask = ((uint64_t)1 << __ffs(vfio_pgsize_bitmap(iommu))) - 1;
>>   
>> @@ -1041,6 +1043,7 @@ static int vfio_dma_do_unmap(struct vfio_iommu *iommu,
>>   			ret = -EINVAL;
>>   			goto unlock;
>>   		}
>> +
>>   		dma = vfio_find_dma(iommu, unmap->iova + unmap->size - 1, 0);
>>   		if (dma && dma->iova + dma->size != unmap->iova + unmap->size) {
>>   			ret = -EINVAL;
>> @@ -1048,6 +1051,22 @@ static int vfio_dma_do_unmap(struct vfio_iommu *iommu,
>>   		}
>>   	}
>>   
>> +	if ((unmap->flags & VFIO_DMA_UNMAP_FLAG_GET_DIRTY_BITMAP) &&
>> +	     iommu->dirty_page_tracking) {
> 
> Why do we even accept VFIO_DMA_UNMAP_FLAG_GET_DIRTY_BITMAP when not
> dirty page tracking rather than returning -EINVAL?  It would simplify
> things here to reject it at the ioctl and silently ignoring a flag is
> rarely if ever the right approach.
> 
>> +		final_bitmap = kvzalloc(bitmap->size, GFP_KERNEL);
>> +		if (!final_bitmap) {
>> +			ret = -ENOMEM;
>> +			goto unlock;
>> +		}
>> +
>> +		temp_bitmap = kvzalloc(bitmap->size, GFP_KERNEL);
>> +		if (!temp_bitmap) {
>> +			ret = -ENOMEM;
>> +			kfree(final_bitmap);
>> +			goto unlock;
>> +		}
> 
> YIKES!  So the user can instantly trigger the kernel to internally
> allocate 2 x 256MB, regardless of how much they can actually map.
> 

That is worst case senario. I don't think ideally that will ever hit. 
More comment below regarding this.

>> +	}
>> +
>>   	while ((dma = vfio_find_dma(iommu, unmap->iova, unmap->size))) {
>>   		if (!iommu->v2 && unmap->iova > dma->iova)
>>   			break;
>> @@ -1058,6 +1077,24 @@ static int vfio_dma_do_unmap(struct vfio_iommu *iommu,
>>   		if (dma->task->mm != current->mm)
>>   			break;
>>   
>> +		if ((unmap->flags & VFIO_DMA_UNMAP_FLAG_GET_DIRTY_BITMAP) &&
>> +		     iommu->dirty_page_tracking) {
>> +			unsigned long pgshift = __ffs(bitmap->pgsize);
>> +			unsigned int npages = dma->size >> pgshift;
>> +			unsigned int shift;
>> +
>> +			vfio_iova_dirty_bitmap(iommu, dma->iova, dma->size,
>> +					bitmap->pgsize, (u64 *)temp_bitmap);
> 
> vfio_iova_dirty_bitmap() takes a __user bitmap, we're doing
> copy_to_user() on a kernel allocated buffer???
> 

Actually, there is no need to call vfio_iova_dirty_bitmap(), dma pointer 
is known here and since its getting unmapped, there is no need to 
repopulate bitmap. Removing vfio_iova_dirty_bitmap() and changing it as 
below:

if (unmap->flags & VFIO_DMA_UNMAP_FLAG_GET_DIRTY_BITMAP) {
     unsigned long pgshift = __ffs(bitmap->pgsize);
     unsigned int npages = dma->size >> pgshift;
     unsigned int bitmap_size = DIRTY_BITMAP_BYTES(npages);
     unsigned int shift = (dma->iova - unmap->iova) >>
                                             pgshift;
     /*
      * mark all pages dirty if all pages are pinned and
      * mapped.
      */
     if (dma->iommu_mapped)
         bitmap_set(temp_bitmap, 0, npages);
     else
         memcpy(temp_bitmap, dma->bitmap, bitmap_size);

     if (shift)
         bitmap_shift_left(temp_bitmap, temp_bitmap,
                           shift, npages);
     bitmap_or(final_bitmap, final_bitmap, temp_bitmap,
               shift + npages);
     memset(temp_bitmap, 0, bitmap->size);
}

>> +
>> +			shift = (dma->iova - unmap->iova) >> pgshift;
>> +			if (shift)
>> +				bitmap_shift_left(temp_bitmap, temp_bitmap,
>> +						  shift, npages);
>> +			bitmap_or(final_bitmap, final_bitmap, temp_bitmap,
>> +				  shift + npages);
>> +			memset(temp_bitmap, 0, bitmap->size);
>> +		}
> 
> It seems like if the per vfio_dma dirty bitmap was oversized by a long
> that we could shift it in place, then we'd only need one working bitmap
> buffer and we could size that to fit the vfio_dma (or the largest
> vfio_dma if we don't want to free and re-alloc for each vfio_dma).
> We'd need to do more copy_to/from_user()s, but we'd also avoid copying
> between sparse mappings (user zero'd bitmap required) and we'd have a
> far more reasonable memory usage.  Thanks,
>

I thought about it, but couldn't optimize to use one bitmap buffer.
This case will only hit during migration with vIOMMU enabled.
Can we keep these 2 bitmap buffers for now and optimize it later?

Thanks,
Kirti