Re: [PATCH v2 1/3] shared-mem: introduce page alignment restrictions

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Mar 31, 2025 at 09:40:37AM -0700, Sergio Lopez Pascual wrote:
> "Michael S. Tsirkin" <mst@redhat.com> writes:
> 
> > On Mon, Mar 31, 2025 at 11:05:46AM -0400, Sergio Lopez wrote:
> >> Add a subsection for page alignment restrictions.
> >>
> >> Signed-off-by: Sergio Lopez <slp@redhat.com>
> >> ---
> >>  shared-mem.tex | 6 ++++++
> >>  1 file changed, 6 insertions(+)
> >>
> >> diff --git a/shared-mem.tex b/shared-mem.tex
> >> index 6e6f6c4..2021083 100644
> >> --- a/shared-mem.tex
> >> +++ b/shared-mem.tex
> >> @@ -34,6 +34,12 @@ \subsection{Addressing within regions}\label{sec:Basic Facilities of a Virtio De
> >>  The \field{shmid} may be explicit or may be inferred from the
> >>  context of the reference.
> >>
> >> +\subsection{Page alignment restrictions}\label{sec:Basic Facilities of a Virtio Device / Shared Memory Regions / Page alignment restrictions}
> >> +
> >> +When requesting the device to map memory into a shared memory region
> >> +the driver MUST obtain the page size information from the transport
> >> +and honor the page alignment constrains derived from that page size.
> >> +
> >
> > will make existing drivers autmatically incompliant with new devices.
> >
> > we generally avoid things like this.
> 
> This is the reason why in v1 this was gated behind
> VIRTIO_F_SHM_PAGE_SIZE. But from the discussion on that thread, I
> understood that, at least for the PCI transport, it was preferred to
> have the page_shift field unconditionally:
> 
> https://lore.kernel.org/virtio-comment/CY8PR12MB71950314C5683EB7CFF0815BDCCA2@CY8PR12MB7195.namprd12.prod.outlook.com/
> 
> Did I misunderstand the conclusions?
> 
> Sergio.

I think Parav missed this compatibility implication when he said:

	So to obtain a free information, feature bit is not necessary, this can be self-
	described in the pci capability itself.

Parav, this is about the MUST requirement referring to honoring
alignment, not about obtaining the information which indeed can
be done without.


-- 
MST