Re: [PATCH 1/5] vfio/iommu_type1: Fixes vfio_dma_populate_bitmap to avoid dirty lose

[Alex Williamson <alex.williamson@redhat.com>]

On Wed, 13 Jan 2021 18:05:43 +0530
Kirti Wankhede <kwankhede@nvidia.com> wrote:

> On 1/13/2021 2:50 AM, Alex Williamson wrote:
> > On Thu, 7 Jan 2021 17:28:57 +0800
> > Keqian Zhu <zhukeqian1@huawei.com> wrote:
> >   
> >> Defer checking whether vfio_dma is of fully-dirty in update_user_bitmap
> >> is easy to lose dirty log. For example, after promoting pinned_scope of
> >> vfio_iommu, vfio_dma is not considered as fully-dirty, then we may lose
> >> dirty log that occurs before vfio_iommu is promoted.
> >>
> >> The key point is that pinned-dirty is not a real dirty tracking way, it
> >> can't continuously track dirty pages, but just restrict dirty scope. It
> >> is essentially the same as fully-dirty. Fully-dirty is of full-scope and
> >> pinned-dirty is of pinned-scope.
> >>
> >> So we must mark pinned-dirty or fully-dirty after we start dirty tracking
> >> or clear dirty bitmap, to ensure that dirty log is marked right away.  
> > 
> > I was initially convinced by these first three patches, but upon
> > further review, I think the premise is wrong.  AIUI, the concern across
> > these patches is that our dirty bitmap is only populated with pages
> > dirtied by pinning and we only take into account the pinned page dirty
> > scope at the time the bitmap is retrieved by the user.  You suppose
> > this presents a gap where if a vendor driver has not yet identified
> > with a page pinning scope that the entire bitmap should be considered
> > dirty regardless of whether that driver later pins pages prior to the
> > user retrieving the dirty bitmap.
> > 
> > I don't think this is how we intended the cooperation between the iommu
> > driver and vendor driver to work.  By pinning pages a vendor driver is
> > not declaring that only their future dirty page scope is limited to
> > pinned pages, instead they're declaring themselves as a participant in
> > dirty page tracking and take responsibility for pinning any necessary
> > pages.  For example we might extend VFIO_IOMMU_DIRTY_PAGES_FLAG_START
> > to trigger a blocking notification to groups to not only begin dirty
> > tracking, but also to synchronously register their current device DMA
> > footprint.  This patch would require a vendor driver to possibly perform
> > a gratuitous page pinning in order to set the scope prior to dirty
> > logging being enabled, or else the initial bitmap will be fully dirty.
> > 
> > Therefore, I don't see that this series is necessary or correct.  Kirti,
> > does this match your thinking?
> >   
> 
> That's correct Alex and I agree with you.
> 
> > Thinking about these semantics, it seems there might still be an issue
> > if a group with non-pinned-page dirty scope is detached with dirty
> > logging enabled.    
> 
> Hot-unplug a device while migration process has started - is this 
> scenario supported?

It's not prevented, it would rely on a userspace policy, right?  The
kernel should do the right thing regardless.  Thanks,

Alex

> > It seems this should in fact fully populate the dirty
> > bitmaps at the time it's removed since we don't know the extent of its
> > previous DMA, nor will the group be present to trigger the full bitmap
> > when the user retrieves the dirty bitmap.  Creating fully populated
> > bitmaps at the time tracking is enabled negates our ability to take
> > advantage of later enlightenment though.  Thanks,
> > 
> > Alex
> >   
> >> Fixes: d6a4c185660c ("vfio iommu: Implementation of ioctl for dirty pages tracking")
> >> Signed-off-by: Keqian Zhu <zhukeqian1@huawei.com>
> >> ---
> >>   drivers/vfio/vfio_iommu_type1.c | 33 ++++++++++++++++++++++-----------
> >>   1 file changed, 22 insertions(+), 11 deletions(-)
> >>
> >> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
> >> index bceda5e8baaa..b0a26e8e0adf 100644
> >> --- a/drivers/vfio/vfio_iommu_type1.c
> >> +++ b/drivers/vfio/vfio_iommu_type1.c
> >> @@ -224,7 +224,7 @@ static void vfio_dma_bitmap_free(struct vfio_dma *dma)
> >>   	dma->bitmap = NULL;
> >>   }
> >>   
> >> -static void vfio_dma_populate_bitmap(struct vfio_dma *dma, size_t pgsize)
> >> +static void vfio_dma_populate_bitmap_pinned(struct vfio_dma *dma, size_t pgsize)
> >>   {
> >>   	struct rb_node *p;
> >>   	unsigned long pgshift = __ffs(pgsize);
> >> @@ -236,6 +236,25 @@ static void vfio_dma_populate_bitmap(struct vfio_dma *dma, size_t pgsize)
> >>   	}
> >>   }
> >>   
> >> +static void vfio_dma_populate_bitmap_full(struct vfio_dma *dma, size_t pgsize)
> >> +{
> >> +	unsigned long pgshift = __ffs(pgsize);
> >> +	unsigned long nbits = dma->size >> pgshift;
> >> +
> >> +	bitmap_set(dma->bitmap, 0, nbits);
> >> +}
> >> +
> >> +static void vfio_dma_populate_bitmap(struct vfio_iommu *iommu,
> >> +				     struct vfio_dma *dma)
> >> +{
> >> +	size_t pgsize = (size_t)1 << __ffs(iommu->pgsize_bitmap);
> >> +
> >> +	if (iommu->pinned_page_dirty_scope)
> >> +		vfio_dma_populate_bitmap_pinned(dma, pgsize);
> >> +	else if (dma->iommu_mapped)
> >> +		vfio_dma_populate_bitmap_full(dma, pgsize);
> >> +}
> >> +
> >>   static int vfio_dma_bitmap_alloc_all(struct vfio_iommu *iommu)
> >>   {
> >>   	struct rb_node *n;
> >> @@ -257,7 +276,7 @@ static int vfio_dma_bitmap_alloc_all(struct vfio_iommu *iommu)
> >>   			}
> >>   			return ret;
> >>   		}
> >> -		vfio_dma_populate_bitmap(dma, pgsize);
> >> +		vfio_dma_populate_bitmap(iommu, dma);
> >>   	}
> >>   	return 0;
> >>   }
> >> @@ -987,13 +1006,6 @@ static int update_user_bitmap(u64 __user *bitmap, struct vfio_iommu *iommu,
> >>   	unsigned long shift = bit_offset % BITS_PER_LONG;
> >>   	unsigned long leftover;
> >>   
> >> -	/*
> >> -	 * mark all pages dirty if any IOMMU capable device is not able
> >> -	 * to report dirty pages and all pages are pinned and mapped.
> >> -	 */
> >> -	if (!iommu->pinned_page_dirty_scope && dma->iommu_mapped)
> >> -		bitmap_set(dma->bitmap, 0, nbits);
> >> -
> >>   	if (shift) {
> >>   		bitmap_shift_left(dma->bitmap, dma->bitmap, shift,
> >>   				  nbits + shift);
> >> @@ -1019,7 +1031,6 @@ static int vfio_iova_dirty_bitmap(u64 __user *bitmap, struct vfio_iommu *iommu,
> >>   	struct vfio_dma *dma;
> >>   	struct rb_node *n;
> >>   	unsigned long pgshift = __ffs(iommu->pgsize_bitmap);
> >> -	size_t pgsize = (size_t)1 << pgshift;
> >>   	int ret;
> >>   
> >>   	/*
> >> @@ -1055,7 +1066,7 @@ static int vfio_iova_dirty_bitmap(u64 __user *bitmap, struct vfio_iommu *iommu,
> >>   		 * pages which are marked dirty by vfio_dma_rw()
> >>   		 */
> >>   		bitmap_clear(dma->bitmap, 0, dma->size >> pgshift);
> >> -		vfio_dma_populate_bitmap(dma, pgsize);
> >> +		vfio_dma_populate_bitmap(iommu, dma);
> >>   	}
> >>   	return 0;
> >>   }  
> >   
> 

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