Re: [PATCH v2 11/20] vfio/common: Add device dirty page tracking start/stop

[Jason Gunthorpe]

On Wed, Feb 22, 2023 at 03:40:43PM -0700, Alex Williamson wrote:
> > +    /*
> > +     * DMA logging uAPI guarantees to support at least num_ranges that 
> > fits into
> > +     * a single host kernel page. To be on the safe side, use this as a 
> > limit
> > +     * from which to merge to a single range.
> > +     */
> > +    max_ranges = qemu_real_host_page_size() / sizeof(*ranges);
> > +    cur_ranges = iova_tree_nnodes(container->mappings);
> > +    control->num_ranges = (cur_ranges <= max_ranges) ? cur_ranges : 1;
> 
> This makes me suspicious that we're implementing to the characteristics
> of a specific device rather than strictly to the vfio migration API.
> Are we just trying to avoid the error handling to support the try and
> fall back to a single range behavior?

This was what we agreed to when making the kernel patches. Userspace
is restricted to send one page of range list to the kernel, and the
kernel will always adjust that to whatever smaller list the device needs.

We added this limit only because we don't want to have a way for
userspace to consume a lot of kernel memory.

See LOG_MAX_RANGES in vfio_main.c

If qemu is viommu mode and it has a huge number of ranges then it must
cut it down before passing things to the kernel.

Jason