On Fri, Jan 17, 2020 at 01:52:44PM +0100, Paolo Bonzini wrote:
> On 16/01/20 21:24, Dr. David Alan Gilbert (git) wrote:
> > + if (dev->vhost_ops->backend_type == VHOST_BACKEND_TYPE_USER) {
> > + /* Round the section to it's page size */
> > + /* First align the start down to a page boundary */
> > + size_t mrs_page = qemu_ram_pagesize(mrs_rb);
> > + uint64_t alignage = mrs_host & (mrs_page - 1);
> > + if (alignage) {
> > + mrs_host -= alignage;
> > + mrs_size += alignage;
> > + mrs_gpa -= alignage;
> > + }
> > + /* Now align the size up to a page boundary */
> > + alignage = mrs_size & (mrs_page - 1);
> > + if (alignage) {
> > + mrs_size += mrs_page - alignage;
> > + }
> > + trace_vhost_region_add_section_aligned(section->mr->name, mrs_gpa,
> > mrs_size,
> > + mrs_host);
> > + }
>
> Ok, now I understand!
>
> So it seems to me that the vhost-user protocol is deficient, it should
> have had two different fields for the region size and the region
> alignment (so that mmap does not fail). But I guess that's just yet
> another thing to remember for vhost-user v2.
We don't really need v2 just to add a field. Compatibility is maintained
using feature bits. Adding that is a subject for another patch.
But I'm not sure I understand why does remote need to know about alignment.
This patch seems to handle it locally ...
> I would add a comment to explain why the alignment is needed in the
> first place, but this fix is certainly much more pleasant. Thanks very
> much.
>
> Reviewed-by: Paolo Bonzini <pbonz...@redhat.com>
>
> Paolo
