Re: [PATCH RFC v2 01/16] vfio-user: introduce vfio-user protocol specification

[John Johnson]

> On Aug 17, 2021, at 7:04 PM, Alex Williamson <alex.williamson@redhat.com> 
> wrote:
> 
> 
> The address/size paradigm falls into the same issues as the vfio kernel
> interface where we can't map or unmap the entire 64-bit address space,
> ie. size is limited to 2^64 - 1.  The kernel interface also requires
> PAGE_SIZE granularity for the DMA, which means the practical limit is
> 2^64 - PAGE_SIZE.  If we had a redo on the kernel interface we'd use
> start/end so we can express a size of (end - start + 1).
> 
> Is following the vfio kernel interface sufficiently worthwhile for
> compatibility to incur this same limitation?  I don't recall if we've
> already discussed this, but perhaps worth a note in this design doc if
> similarity to the kernel interface is being favored here.  See for
> example QEMU commit 1b296c3def4b ("vfio: Don't issue full 2^64 unmap").
> Thanks,
> 


        I’d prefer to stay as close to the kernel i/f as we can.
An earlier version of the spec used a vhost-user derived structure
for MAP & UNMAP.  This made it more difficult to add the bitmap
field when vfio added migration capability, so we switched to the
ioctl() structure.

        It looks like vfio_dma_unmap() takes a 64b ‘size’ arg
(ram_addr_t)  How did you unmap an entire 64b address space?  The
comment there mentions a bug where iova+size wraps the end of the
64b space.

                                                        JJ