Re: [PATCH v15 Kernel 4/7] vfio iommu: Implementation of ioctl for dirty pages tracking.

[Kirti Wankhede <kwankhede@nvidia.com>]

On 3/20/2020 11:31 PM, Alex Williamson wrote:
> On Fri, 20 Mar 2020 23:19:14 +0530
> Kirti Wankhede <kwankhede@nvidia.com> wrote:
> 
>> On 3/20/2020 4:27 AM, Alex Williamson wrote:
>>> On Fri, 20 Mar 2020 01:46:41 +0530
>>> Kirti Wankhede <kwankhede@nvidia.com> wrote:
>>>    

<snip>

>>>> +static int vfio_iova_dirty_bitmap(struct vfio_iommu *iommu, dma_addr_t iova,
>>>> +				  size_t size, uint64_t pgsize,
>>>> +				  u64 __user *bitmap)
>>>> +{
>>>> +	struct vfio_dma *dma;
>>>> +	unsigned long pgshift = __ffs(pgsize);
>>>> +	unsigned int npages, bitmap_size;
>>>> +
>>>> +	dma = vfio_find_dma(iommu, iova, 1);
>>>> +
>>>> +	if (!dma)
>>>> +		return -EINVAL;
>>>> +
>>>> +	if (dma->iova != iova || dma->size != size)
>>>> +		return -EINVAL;
>>>> +
>>>> +	npages = dma->size >> pgshift;
>>>> +	bitmap_size = DIRTY_BITMAP_BYTES(npages);
>>>> +
>>>> +	/* mark all pages dirty if all pages are pinned and mapped. */
>>>> +	if (dma->iommu_mapped)
>>>> +		bitmap_set(dma->bitmap, 0, npages);
>>>> +
>>>> +	if (copy_to_user((void __user *)bitmap, dma->bitmap, bitmap_size))
>>>> +		return -EFAULT;
>>>
>>> We still need to reset the bitmap here, clearing and re-adding the
>>> pages that are still pinned.
>>>
>>> https://lore.kernel.org/kvm/20200319070635.2ff5db56@x1.home/
>>>    
>>
>> I thought you agreed on my reply to it
>> https://lore.kernel.org/kvm/31621b70-02a9-2ea5-045f-f72b671fe703@nvidia.com/
>>
>>   > Why re-populate when there will be no change since
>>   > vfio_iova_dirty_bitmap() is called holding iommu->lock? If there is any
>>   > pin request while vfio_iova_dirty_bitmap() is still working, it will
>>   > wait till iommu->lock is released. Bitmap will be populated when page is
>>   > pinned.
> 
> As coded, dirty bits are only ever set in the bitmap, never cleared.
> If a page is unpinned between iterations of the user recording the
> dirty bitmap, it should be marked dirty in the iteration immediately
> after the unpinning and not marked dirty in the following iteration.
> That doesn't happen here.  We're reporting cumulative dirty pages since
> logging was enabled, we need to be reporting dirty pages since the user
> last retrieved the dirty bitmap.  The bitmap should be cleared and
> currently pinned pages re-added after copying to the user.  Thanks,
> 

Does that mean, we have to track every iteration? do we really need that 
tracking?

Generally the flow is:
- vendor driver pin x pages
- Enter pre-copy-phase where vCPUs are running - user starts dirty pages 
tracking, then user asks dirty bitmap, x pages reported dirty by 
VFIO_IOMMU_DIRTY_PAGES ioctl with _GET flag
- In pre-copy phase, vendor driver pins y more pages, now bitmap 
consists of x+y bits set
- In pre-copy phase, vendor driver unpins z pages, but bitmap is not 
updated, so again bitmap consists of x+y bits set.
- Enter in stop-and-copy phase, vCPUs are stopped, mdev devices are stopped
- user asks dirty bitmap - Since here vCPU and mdev devices are stopped, 
pages should not get dirty by guest driver or the physical device. 
Hence, x+y dirty pages would be reported.

I don't think we need to track every iteration of bitmap reporting.

Thanks,
Kirti