Re: [virtio-dev] [PATCH] split-ring: Demand that a device must not change descriptor entries

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

On Mon, Nov 11, 2019 at 06:26:58PM +0100, Jan Kiszka wrote:
> On 11.11.19 18:17, Rob Miller wrote:
> > what is trying to be solved here? There is a reason why this is allowed
> > as some vendors update the table when using RX_MERABLE_BUFFERs &
> > F_IN_ORDER features
> 
> OK, good to know. Which fields are updated then? Under which conditions (ie.
> in which states of the device)?
> 
> The background is in the link below. My goal was to find a simple way
> declaring all fields read-only for the device to avoid problems with new
> implementations. But it looks like it's more complicated...
> 
> Jan

No answer yet. Rob I see the following options:
- proceed with the ballot as is ignoring your comment. Maybe it's all a
  misunderstanding? It could be the comment was referring to the
  packed ring, where this patch refers to the split ring.
- make it weaker: MUST if PLATFORM_ACCESS is negotiated, and
  SHOULD if it isn't.

> > 
> > Rob Miller
> > rob.miller@broadcom.com <mailto:rob.miller@broadcom.com>
> > (919)721-3339
> > 
> > 
> > On Mon, Nov 11, 2019 at 12:13 PM Jan Kiszka <jan.kiszka@siemens.com
> > <mailto:jan.kiszka@siemens.com>> wrote:
> > 
> >     From: Jan Kiszka <jan.kiszka@siemens.com
> >     <mailto:jan.kiszka@siemens.com>>
> > 
> >     So far the spec only indirectly says that a descriptor table entry is
> >     not modified by a device when processing it. Make this explicit by
> >     adding it as normative requirement. Existing drivers already depend on
> >     this.
> > 
> >     See also
> >     https://lists.oasis-open.org/archives/virtio-dev/201910/msg00057.html.
> > 
> >     Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com
> >     <mailto:jan.kiszka@siemens.com>>
> >     ---
> >       split-ring.tex | 2 +-
> >       1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> >     diff --git a/split-ring.tex b/split-ring.tex
> >     index 123ac9f..bfef62d 100644
> >     --- a/split-ring.tex
> >     +++ b/split-ring.tex
> >     @@ -217,7 +217,7 @@ \subsection{The Virtqueue Descriptor
> >     Table}\label{sec:Basic Facilities of a Virt
> >       \devicenormative{\subsubsection}{The Virtqueue Descriptor
> >     Table}{Basic Facilities of a Virtio Device / Virtqueues / The
> >     Virtqueue Descriptor Table}
> >       A device MUST NOT write to a device-readable buffer, and a device
> >     SHOULD NOT
> >       read a device-writable buffer (it MAY do so for debugging or
> >     diagnostic
> >     -purposes).
> >     +purposes). A device MUST NOT write to any descriptor table entry.
> > 
> >       \drivernormative{\subsubsection}{The Virtqueue Descriptor
> >     Table}{Basic Facilities of a Virtio Device / Virtqueues / The
> >     Virtqueue Descriptor Table}
> >       Drivers MUST NOT add a descriptor chain longer than $2^{32}$ bytes
> >     in total;
> >     --     2.16.4
> > 
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> 
> -- 
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> Corporate Competence Center Embedded Linux
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