RE: [PATCH V2 vfio 03/11] vfio: Introduce DMA logging uAPIs

["Tian, Kevin" <kevin.tian@intel.com>]

> From: Jason Gunthorpe <jgg@nvidia.com>
> Sent: Monday, July 25, 2022 10:33 PM
> 
> On Mon, Jul 25, 2022 at 07:20:16AM +0000, Tian, Kevin wrote:
> 
> > I got that point. But my question is slightly different.
> >
> > A practical flow would like below:
> >
> > 1) Qemu first requests to start dirty tracking in 4KB page size.
> >    Underlying trackers may start tracking in 4KB, 256KB, 2MB,
> >    etc. based on their own constraints.
> >
> > 2) Qemu then reads back dirty reports in a shared bitmap in
> >    4KB page size. All trackers must update dirty bitmap in 4KB
> >    granular regardless of the actual size each tracker selects.
> >
> > Is there a real usage where Qemu would want to attempt
> > different page sizes between above two steps?
> 
> If you multi-thread the tracker reads it will be efficient to populate
> a single bitmap and then copy that single bitmapt to the dirty
> transfer. In this case you want the page size conversion.
> 
> If qemu is just going to read sequentially then perhaps it doesn't.
> 
> But forcing a fixed page size just denies userspace this choice, and
> it doesn't make the kernel any simpler because the kernel always must
> have this code to adapt different page sizes to support the real iommu
> with huge pages/etc.
> 

OK, make sense.