From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Sender: List-Post: List-Help: List-Unsubscribe: List-Subscribe: Received: from lists.oasis-open.org (oasis-open.org [10.110.1.242]) by lists.oasis-open.org (Postfix) with ESMTP id DB3EE9866EC for ; Sat, 19 Mar 2022 10:12:34 +0000 (UTC) From: Christian Schoenebeck Date: Sat, 19 Mar 2022 11:12:30 +0100 Message-ID: <15118124.lv8FRMpzUk@silver> In-Reply-To: <20220318170625.0d2be174.pasic@linux.ibm.com> References: <4735344.EBYxvr1mta@silver> <11686863.Z9n2BMzBuM@silver> <20220318170625.0d2be174.pasic@linux.ibm.com> MIME-Version: 1.0 Subject: Re: [virtio-comment] Re: [PATCH v3 4/4] Add CCW configuration field "indirect_num" Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="iso-8859-1" To: Halil Pasic Cc: Cornelia Huck , virtio-comment@lists.oasis-open.org, Stefan Hajnoczi , Greg Kurz , Dominique Martinet List-ID: On Freitag, 18. M=E4rz 2022 17:06:25 CET Halil Pasic wrote: > On Fri, 18 Mar 2022 12:02:31 +0100 >=20 > Christian Schoenebeck wrote: > > On Donnerstag, 17. M=E4rz 2022 15:12:42 CET Cornelia Huck wrote: > > > On Wed, Mar 16 2022, Christian Schoenebeck = =20 wrote: > > > > This new CCW configuration field allows to negotiate a more fine > > > > graded maximum lenght of indirect descriptor chains. > > > >=20 > > > > Bump CCW virtio revision to 3 and make the existence of this new > > > > field "indirect_num" dependant on revision 3. > > > >=20 > > > > Fixes: https://github.com/oasis-tcs/virtio-spec/issues/122 > > > > Signed-off-by: Christian Schoenebeck > > > > --- > > > >=20 > > > > content.tex | 14 +++++++++++++- > > > > 1 file changed, 13 insertions(+), 1 deletion(-) > > >=20 > > > (...) > > >=20 > > > > @@ -2581,12 +2583,17 @@ \subsubsection{Configuring a > > > > Virtqueue}\label{sec:Virtio Transport Options / Vir> > > > >=20 > > > > struct vq_config_block { > > > > =20 > > > > be16 index; > > > > be16 max_num; > > > >=20 > > > > + be32 indirect_num; /* since virtio-ccw rev. 3 */ > > >=20 > > > Maybe make it max_indirect_num (to mirror max_num?) > >=20 > > Right, that name appears indeed more appropriate here. > >=20 > > > > }; > > > > \end{lstlisting} > > > > =20 > > > > The requested number of buffers for queue \field{index} is returne= d > > > > in > > > > \field{max_num}. > > > >=20 > > > > +Since revision 3, \field{indirect_num} exists, which is supposed t= o > > > > reflect the +Queue Indirect Size (i.e. the maximum length of indire= ct > > > > descriptor tables) +supported by device for this queue. > > >=20 > > > It still depends on the feature flag, though. > > >=20 > > > Maybe > > >=20 > > > "If revision 2 or lower is set, struct vq_config_block extends up to > > > \field{max_num}. > > >=20 > > > If revision 3 or higher is set, struct vq_config_block extends to > > > \field{max_indirect_num}. If VIRTIO_RING_F_INDIRECT_SIZE is negotiate= d, > > > \field{indirect_max_num} contains the maximum Queue Indirect Size > > > (i.e. the maximum length of indirect descriptor tables) supported by = the > > > device for this queue; otherwise, its contents are unpredictable." > >=20 > > Maybe rather something like this instead: > >=20 > > "Actual size of struct vq_config_block depends on the virtio-ccw revisi= on. > >=20 > > If revision 2 or lower is set, struct vq_config_block extends up to > > *including* \field{max_num}. > >=20 > > If revision 3 or higher is set, struct vq_config_block extends up to > > *including* \field{max_indirect_num}. If VIRTIO_RING_F_INDIRECT_SIZE is > > negotiated, \field{indirect_max_num} contains the maximum Queue Indirec= t > > Size (i.e. the maximum length of indirect descriptor tables) supported = by > > the device for this queue; otherwise, its contents are *undefined*." >=20 > I agree that the "including" is important, but I'm not sure about the > "its contents are undefined". I don't really understand why should we use > a plural here. What speaks against specifying that in SHOULD be stored > as 0 by the device, and MUST be ignored by the driver? Both solutions would be viable. Personally I would just use something like= =20 "Should be zero" if there is a value in recommending that, but I don't see = a=20 value in recommending to set something to zero and at the same time requiri= ng=20 to not access it in the first place. > Currently we say that \field{max_indirect_num} exists like a be32 field > even if VIRTIO_RING_F_INDIRECT_SIZE is not negotiated. Which kind of > implies that at least type invariants should hold. Of course, there is > none here (i.e. every bits value is also a be32 value), but for something > like an enum interesting corner cases can pop up. I can't follow you on that one. What has that do with enums in this case? Anyway, I won't persist on my suggestion to use the (IMO more compact form)= =20 "undefined". If you guys prefer the more specific solution "SHOULD be 0 and= =20 MUST not be accessed" then I will go that way. 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