Re: [PATCH v12 Kernel 4/7] vfio iommu: Implementation of ioctl to for dirty pages tracking.

[Yan Zhao <yan.y.zhao@intel.com>]

On Tue, Feb 11, 2020 at 03:44:54AM +0800, Alex Williamson wrote:
> On Mon, 10 Feb 2020 04:49:54 -0500
> Yan Zhao <yan.y.zhao@intel.com> wrote:
> 
> > On Sat, Feb 08, 2020 at 03:42:31AM +0800, Kirti Wankhede wrote:
> > > VFIO_IOMMU_DIRTY_PAGES ioctl performs three operations:
> > > - Start pinned and unpinned pages tracking while migration is active
> > > - Stop pinned and unpinned dirty pages tracking. This is also used to
> > >   stop dirty pages tracking if migration failed or cancelled.
> > > - Get dirty pages bitmap. This ioctl returns bitmap of dirty pages, its
> > >   user space application responsibility to copy content of dirty pages
> > >   from source to destination during migration.
> > > 
> > > To prevent DoS attack, memory for bitmap is allocated per vfio_dma
> > > structure. Bitmap size is calculated considering smallest supported page
> > > size. Bitmap is allocated when dirty logging is enabled for those
> > > vfio_dmas whose vpfn list is not empty or whole range is mapped, in
> > > case of pass-through device.
> > > 
> > > There could be multiple option as to when bitmap should be populated:
> > > * Polulate bitmap for already pinned pages when bitmap is allocated for
> > >   a vfio_dma with the smallest supported page size. Updates bitmap from
> > >   page pinning and unpinning functions. When user application queries
> > >   bitmap, check if requested page size is same as page size used to
> > >   populated bitmap. If it is equal, copy bitmap. But if not equal,
> > >   re-populated bitmap according to requested page size and then copy to
> > >   user.
> > >   Pros: Bitmap gets populated on the fly after dirty tracking has
> > >         started.
> > >   Cons: If requested page size is different than smallest supported
> > >         page size, then bitmap has to be re-populated again, with
> > >         additional overhead of allocating bitmap memory again for
> > >         re-population of bitmap.
> > > 
> > > * Populate bitmap when bitmap is queried by user application.
> > >   Pros: Bitmap is populated with requested page size. This eliminates
> > >         the need to re-populate bitmap if requested page size is
> > >         different than smallest supported pages size.
> > >   Cons: There is one time processing time, when bitmap is queried.
> > > 
> > > I prefer later option with simple logic and to eliminate over-head of
> > > bitmap repopulation in case of differnt page sizes. Later option is
> > > implemented in this patch.
> > > 
> > > Signed-off-by: Kirti Wankhede <kwankhede@nvidia.com>
> > > Reviewed-by: Neo Jia <cjia@nvidia.com>
> > > ---
> > >  drivers/vfio/vfio_iommu_type1.c | 299 ++++++++++++++++++++++++++++++++++++++--
> > >  1 file changed, 287 insertions(+), 12 deletions(-)
> > > 
> > > diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
> > > index d386461e5d11..df358dc1c85b 100644
> > > --- a/drivers/vfio/vfio_iommu_type1.c
> > > +++ b/drivers/vfio/vfio_iommu_type1.c
> [snip]
> > > @@ -830,6 +924,113 @@ static unsigned long vfio_pgsize_bitmap(struct vfio_iommu *iommu)
> > >  	return bitmap;
> > >  }
> > >  
> > > +static int vfio_iova_dirty_bitmap(struct vfio_iommu *iommu, dma_addr_t iova,
> > > +				  size_t size, uint64_t pgsize,
> > > +				  unsigned char __user *bitmap)
> > > +{
> > > +	struct vfio_dma *dma;
> > > +	dma_addr_t i = iova, iova_limit;
> > > +	unsigned int bsize, nbits = 0, l = 0;
> > > +	unsigned long pgshift = __ffs(pgsize);
> > > +
> > > +	while ((dma = vfio_find_dma(iommu, i, pgsize))) {
> > > +		int ret, j;
> > > +		unsigned int npages = 0, shift = 0;
> > > +		unsigned char temp = 0;
> > > +
> > > +		/* mark all pages dirty if all pages are pinned and mapped. */
> > > +		if (dma->iommu_mapped) {
> > > +			iova_limit = min(dma->iova + dma->size, iova + size);
> > > +			npages = iova_limit/pgsize;
> > > +			bitmap_set(dma->bitmap, 0, npages);  
> > for pass-through devices, it's not good to always return all pinned pages as
> > dirty. could it also call vfio_pin_pages to track dirty pages? or any
> > other interface provided to do that?
> 
> See patch 7/7.  Thanks,
>
hi Alex and Kirti,
for pass-through devices, though patch 7/7 enables the vendor driver to
set dirty pages by calling vfio_pin_pages, however, its overhead is much
higher than the previous way of generating a bitmap directly to user.
And it also requires pass-through device vendor driver to track guest
operations to know when to call vfio_pin_pages.
There are still use cases like a pass-through device is able to track
dirty pages in its hardware buffer, so is there a way for it pass its
dirty bitmap to user?

Thanks
Yan