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 028B7983F88 for ; Sun, 20 Mar 2022 15:17:55 +0000 (UTC) From: Christian Schoenebeck Date: Sun, 20 Mar 2022 16:17:48 +0100 Message-ID: <3695921.aWz9qjfz3Z@silver> In-Reply-To: <20220320093409-mutt-send-email-mst@kernel.org> References: <4735344.EBYxvr1mta@silver> <149799798.HqrVOTAGB4@silver> <20220320093409-mutt-send-email-mst@kernel.org> MIME-Version: 1.0 Subject: Re: [virtio-comment] [PATCH v3 1/4] Add VIRTIO_RING_F_INDIRECT_SIZE Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="iso-8859-1" To: "Michael S. Tsirkin" Cc: virtio-comment@lists.oasis-open.org, Cornelia Huck , Stefan Hajnoczi , Greg Kurz , Dominique Martinet , Halil Pasic List-ID: On Sonntag, 20. M=E4rz 2022 14:55:59 CET Michael S. Tsirkin wrote: > On Sun, Mar 20, 2022 at 02:32:23PM +0100, Christian Schoenebeck wrote: > > On Sonntag, 20. M=E4rz 2022 13:31:51 CET Michael S. Tsirkin wrote: > > > On Sat, Mar 19, 2022 at 01:00:28PM +0100, Christian Schoenebeck wrote= : > > > > On Samstag, 19. M=E4rz 2022 10:33:49 CET Michael S. Tsirkin wrote: > > > > > On Wed, Mar 16, 2022, 15:47 Christian Schoenebeck > > > > > > > > > >=20 > > > > > wrote: > > > > > > This new feature flag allows to decouple the maximum amount of > > > > > > descriptors in indirect descriptor tables from the Queue Size. > > > > >=20 > > > > > if we are extending these limits, I suggest reusing the feature f= lag > > > > > to > > > > > also add a limit on total s/g list size. making it separate from > > > > > queue > > > > > size > > > > > was requested a while ago. > > > >=20 > > > > What do you mean with "total s/g list size"? The maximum bulk data > > > > size > > > > per > > > > message? > > > > Sum of both in and out s/g lists' bulk data or only for one of them= ? > > > > Or maximum size of exactly only one memory segment? > > >=20 > > > I don't really know what does "bulk data size" mean. Suggest we use > > > terminology from the spec. A buffer includes a group of direct and/or > > > indirect descriptors, in turn indirect descriptors point to direct > > > descriptors. > >=20 > > I already described why I think it makes sense not calling it "buffer" = in > > this particular context. So I am against changing this to "buffer". >=20 > Well spec just defines what buffers are. > If you are using a different term then you need to define it in the > spec. Sorry, but that sounds like nitpicking to me. From split-ring.tex: "When the driver wants to send a buffer to the device, it fills in a *slot* in the descriptor table (or chains several together)" So a "slot in the descriptor table" does not need further specification, bu= t=20 the term "vring slot" does? > > If other people > > support your position then I'll change it though of course. > >=20 > > About "bulk data size": "Bulk data" is the user data actually being use= d > > on > > application/driver level, i.e. above virtio level, and "Bulk data size" > > the > > size of that data. See ASCII illustration here: > > https://github.com/oasis-tcs/virtio-spec/issues/122 > >=20 > > The terminology "bulk data" is already used in the spec already BTW. >=20 > It does not refer to anything specific though, just generally > to vqs for passing lots of data as opposed to config space used > to pass small amount of data. Which is telling pretty much precise enough what it is, at least IMO. > > > What has been requested a while is ability to limit per vq the > > > # of direct descriptors in a buffer. > >=20 > > So in cases where indirect descriptors are *not* used. This series is > > about > > indirect descriptors only though. >=20 > No, in all cases. ? =20 > > > Since IIUC what you want to do is allow more descriptors > > > than VQ size, then one way to achieve that is just to have > > > a per VQ limit on descriptor size and have that limit > VQ size. > >=20 > > Sorry, I can't follow you. What do you mean with "descriptor size"? > > For me a > > descriptor has a predefined constant struct size. You mean the size of = one > > memory segment referenced by one indirect descriptor? And why would it = be > > better than what this series suggests? >=20 > My bad. What I meant is a "per VQ limit on # of direct descriptors > per buffer". Which is out of the scope of what this series was about. > > > Another thing related is that people wanted to block indirect > > > descriptors for some VQs. Not yet sure how to combine that > > > with this proposal, worth thinking about. > >=20 > > This series allows both, increasing *and* decreasing the number of > > indirect > > descriptors per VQ already. >=20 > I don't see how you block indirect descritors for a queue with this. > Did I miss it? By setting the proposed "Queue Indirect Size" value to zero? > Also I think you missed the fact that > a direct descriptor can point to an indirect one, the > result is that max # of descriptors in a buffer is then: >=20 > queue size - 1 + indirect table size >=20 > I don't see how your proposal limits the # of descriptors > below queue size since guest is never forced to use > indirect. No, I didn't miss that. The suggested changes were about the amount of=20 *indirect* descriptors, not about the amount of *direct* descriptors. The= =20 amount of direct descriptors was still limited to the "Queue Size". I am aware that QEMU currently has a limit per "buffer" which adds the amou= nt=20 of direct descriptors *and* the amount of indirect ones together for that= =20 limit (which I also mentioned from the Github issue summary BTW). Which is = a=20 device specific implementation feature of QEMU and would not stop QEMU thou= gh=20 to handle this correctly by reducing the negotiated "Queue Indirect Size"= =20 value appropriately. > > > > And are you suggesting this should become part of this series alrea= dy? > > >=20 > > > yes since it's touching mostly same areas in the spec. > > : > > :/ Please note that I sent the first draft on this issue already in > > :November > >=20 > > last year, and have not seen any response from your side so far. I > > actually > > assumed we were already at a point where it was just about precise word= ing > > et al., not restarting to redesign everything again from scratch now. > Sorry about that. I'm pretty sure you are. > > > > > > The new term "Queue Indirect Size" is introduced for this purpo= se, > > > > > > which is a transport specific configuration whose negotiation i= s > > > > > > further specified for each transport with subsequent patches. > > > > > >=20 > > > > > > Fixes: https://github.com/oasis-tcs/virtio-spec/issues/122 > > > > > > Signed-off-by: Christian Schoenebeck > > > > > > Reviewed-by: Stefan Hajnoczi > > > > > > --- > > > > > >=20 > > > > > > content.tex | 32 ++++++++++++++++++++++++++++++-- > > > > > > packed-ring.tex | 2 +- > > > > > > split-ring.tex | 8 ++++++-- > > > > > > 3 files changed, 37 insertions(+), 5 deletions(-) > > > > > >=20 > > > > > > diff --git a/content.tex b/content.tex > > > > > > index c6f116c..685525d 100644 > > > > > > --- a/content.tex > > > > > > +++ b/content.tex > > > > > > @@ -99,10 +99,10 @@ \section{Feature Bits}\label{sec:Basic > > > > > > Facilities > > > > > > of a > > > > > > Virtio Device / Feature B > > > > > >=20 > > > > > > \begin{description} > > > > > > \item[0 to 23, and 50 to 127] Feature bits for the specific > > > > > > device > > > > > > type > > > > > >=20 > > > > > > -\item[24 to 40] Feature bits reserved for extensions to the qu= eue > > > > > > and > > > > > > +\item[24 to 41] Feature bits reserved for extensions to the qu= eue > > > > > > and > > > > > >=20 > > > > > > feature negotiation mechanisms > > > > > >=20 > > > > > > -\item[41 to 49, and 128 and above] Feature bits reserved for > > > > > > future > > > > > > extensions. > > > > > > +\item[42 to 49, and 128 and above] Feature bits reserved for > > > > > > future > > > > > > extensions. > > > > > >=20 > > > > > > \end{description} > > > > > > =20 > > > > > > \begin{note} > > > > > >=20 > > > > > > @@ -1051,6 +1051,10 @@ \subsubsection{Common configuration > > > > > > structure > > > > > > layout}\label{sec:Virtio Transport > > > > > >=20 > > > > > > present either a value of 0 or a power of 2 in > > > > > > \field{queue_size}. > > > > > >=20 > > > > > > +If VIRTIO_RING_F_INDIRECT_SIZE has been negotiated, the device > > > > > > MUST > > > > > > provide the > > > > > > +Queue Indirect Size supported by device, which is a transport > > > > > > specific > > > > > > +configuration. It MUST allow the driver to set a lower value. > > > > > > + > > > > > >=20 > > > > > > \drivernormative{\paragraph}{Common configuration structure > > > > > >=20 > > > > > > layout}{Virtio Transport Options / Virtio Over PCI Bus / PCI > > > > > > Device > > > > > > Layout > > > > > > / Common configuration structure layout} > > > > > >=20 > > > > > > The driver MUST NOT write to \field{device_feature}, > > > > > > \field{num_queues}, > > > > > >=20 > > > > > > \field{config_generation}, \field{queue_notify_off} or > > > > > > \field{queue_notify_data}. > > > > > > @@ -6847,6 +6851,30 @@ \chapter{Reserved Feature > > > > > > Bits}\label{sec:Reserved > > > > > > Feature Bits} > > > > > >=20 > > > > > > that the driver can reset a queue individually. > > > > > > See \ref{sec:Basic Facilities of a Virtio Device / Virtqueue= s / > > > > > >=20 > > > > > > Virtqueue Reset}. > > > > > >=20 > > > > > > + \item[VIRTIO_RING_F_INDIRECT_SIZE(41)] This feature indicate= s > > > > > > that > > > > > > the > > > > > > + Queue Indirect Size, i.e. the maximum amount of descriptors = in > > > > > > indirect > > > > > > + descriptor tables, is independent from the Queue Size. > > > > > > + > > > > > > + Without this feature, the Queue Size limits the length of th= e > > > > > > descriptor > > > > > > + chain, including indirect descriptor tables as in > > > > > > \ref{sec:Basic > > > > > > Facilities of > > > > > > + a Virtio Device / Virtqueues / The Virtqueue Descriptor Tabl= e / > > > > > > Indirect > > > > > > + Descriptors}, i.e. both the maximum amount of slots in the > > > > > > vring > > > > > > and > > > > > > the > > > > > > + actual bulk data size transmitted per vring slot. > > > > >=20 > > > > > spect does not call these slots elsewhere. > > > >=20 > > > > Yes, I intentionally used "vring slot" instead of "buffer" as I fin= d > > > > the > > > > latter too vague in this context. A "buffer" can be a memory segmen= t, > > > > a > > > > set of memory segments and what not. "vring slot" OTOH makes it cle= ar > > > > that it is about exactly one, atomic pointer (hence with fixed size= ) > > > > in a > > > > Ring Buffer, as depicted in the ASCII illustration here: > > > >=20 > > > > https://github.com/oasis-tcs/virtio-spec/issues/122 > > > >=20 > > > > The maximum amount of vring slots is therefore the maximum amount o= f > > > > messages that can be emplaced into a Ring Buffer, independent of an= y > > > > "bulk data buffer size". > > > >=20 > > > > > + > > > > >=20 > > > > > > + With this feature enabled, the Queue Size only limits the > > > > > > maximum > > > > > > amount > > > > > > + of slots in the vring, but does not limit the actual bulk da= ta > > > > > > size > > > > > > + being transmitted when indirect descriptors are used. > > > > > > Decoupling > > > > > > these > > > > > > + two configuration parameters this way not only allows much > > > > > > larger > > > > > > bulk > > > > > > data > > > > > > + being transferred per vring slot, but also avoids complicate= d > > > > > > synchronization > > > > > > + mechanisms if the device only supports a very small amount o= f > > > > > > vring > > > > > > slots. Due > > > > > > + to the 16-bit size of a descriptor's "next" field there is > > > > > > still an > > > > > > absolute > > > > > > + limit of $2^{16}$ descriptors per indirect descriptor table. > > > > > > However > > > > > > the > > > > > > + actual maximum amount supported by either device or driver > > > > > > might be > > > > > > less, > > > > > > + and therefore the bus specific Queue Indirect Size value MUS= T > > > > > > additionally > > > > > > + be negotiated if VIRTIO_RING_F_INDIRECT_SIZE was negotiated = to > > > > > > subsequently > > > > > > + negotiate the actual amount of maximum indirect descriptors > > > > > > supported > > > > > > + by both sides. > > > > >=20 > > > > > still not sure what exactly is the value. e.g. in a buffer includ= ing > > > > > indirect and direct descriptors. > > > > >=20 > > > > > + > > > > >=20 > > > > > > \end{description} > > > > > > =20 > > > > > > \drivernormative{\section}{Reserved Feature Bits}{Reserved > > > > > > Feature > > > > > > Bits} > > > > > >=20 > > > > > > diff --git a/packed-ring.tex b/packed-ring.tex > > > > > > index a9e6c16..e26d112 100644 > > > > > > --- a/packed-ring.tex > > > > > > +++ b/packed-ring.tex > > > > > > @@ -195,7 +195,7 @@ \subsection{Scatter-Gather Support} > > > > > >=20 > > > > > > The device limits the number of descriptors in a list through = a > > > > > > transport-specific and/or device-specific value. If not limite= d, > > > > > > the maximum number of descriptors in a list is the virt queue > > > > > >=20 > > > > > > -size. > > > > > > +size unless the VIRTIO_RING_F_INDIRECT_SIZE feature has been > > > > > > negotiated. > > > > > >=20 > > > > > > \subsection{Next Flag: Descriptor Chaining} > > > > > > \label{sec:Packed Virtqueues / Next Flag: Descriptor Chaining} > > > > > >=20 > > > > > > diff --git a/split-ring.tex b/split-ring.tex > > > > > > index de94038..eaa90c3 100644 > > > > > > --- a/split-ring.tex > > > > > > +++ b/split-ring.tex > > > > > > @@ -268,8 +268,12 @@ \subsubsection{Indirect > > > > > > Descriptors}\label{sec:Basic > > > > > > Facilities of a Virtio Devi > > > > > >=20 > > > > > > set the VIRTQ_DESC_F_INDIRECT flag within an indirect descript= or > > > > > > (ie. > > > > > > only > > > > > > one table per descriptor). > > > > > >=20 > > > > > > -A driver MUST NOT create a descriptor chain longer than the Qu= eue > > > > > > Size of > > > > > > -the device. > > > > >=20 > > > > > +If VIRTIO_RING_F_INDIRECT_SIZE has not been negotiated, the driv= er > > > > > MUST > > > > >=20 > > > > > > +NOT create a descriptor chain longer than the Queue Size of th= e > > > > > > device. > > > > > > + > > > > > > +If VIRTIO_RING_F_INDIRECT_SIZE has been negotiated, the number= of > > > > > > +descriptors per indirect descriptor table MUST NOT exceed the > > > > > > negotiated > > > > > > +Queue Indirect Size. > > > > >=20 > > > > > it is not negotiated is it? > > > >=20 > > > > What makes you think it is not negotiated? > >=20 > > Also see my previous question here ^ >=20 > Sorry, what I mean is that you don't define what does negotiation > involve. I think you mean this: >=20 > =09The driver SHOULD write to \field{queue_indirect_size} if its maximum > =09number of descriptors per vring slot is lower than that reported by th= e > =09device. >=20 > but driver can just read the value and that's it - and then the value > that is set by device applies, right? >=20 > If you are going to use terms such as negotiated you need to define what > they mean. In this case I would just say something like > "the value of Queue Indirect Size". Which makes me wonder why you just didn't say that in the first place? And = I=20 don't agree that it wasn't defined, because I actually think I did: + \item[VIRTIO_RING_F_INDIRECT_SIZE(41)] This feature indicates that the + Queue Indirect Size, i.e. the maximum amount of descriptors in indirect + descriptor tables, is independent from the Queue Size." Or is that definition of the new term "Queue Indirect Size" not clear enoug= h=20 to you? > > > > > > A driver MUST NOT set both VIRTQ_DESC_F_INDIRECT and > > > > > > VIRTQ_DESC_F_NEXT > > > > > > in \field{flags}. > > > > > >=20 > > > > > > -- > > > > > > 2.30.2 > > > > > >=20 > > > > > >=20 > > > > > >=20 > > > > > >=20 > > > > > >=20 > > > > > > This publicly archived list offers a means to provide input to = the > > > > > > OASIS Virtual I/O Device (VIRTIO) TC. > > > > > >=20 > > > > > > In order to verify user consent to the Feedback License terms a= nd > > > > > > to minimize spam in the list archive, subscription is required > > > > > > before posting. > > > > > >=20 > > > > > > Subscribe: virtio-comment-subscribe@lists.oasis-open.org > > > > > > Unsubscribe: virtio-comment-unsubscribe@lists.oasis-open.org > > > > > > List help: virtio-comment-help@lists.oasis-open.org > > > > > > List archive: > > > > > > https://lists.oasis-open.org/archives/virtio-comment/ > > > > > > Feedback License: > > > > > > https://www.oasis-open.org/who/ipr/feedback_license.pdf > > > > > > List Guidelines: > > > > > > https://www.oasis-open.org/policies-guidelines/mailing-lists > > > > > > Committee: https://www.oasis-open.org/committees/virtio/ > > > > > > Join OASIS: https://www.oasis-open.org/join/ This publicly archived list offers a means to provide input to the=0D OASIS Virtual I/O Device (VIRTIO) TC.=0D =0D In order to verify user consent to the Feedback License terms and=0D to minimize spam in the list archive, subscription is required=0D before posting.=0D =0D Subscribe: virtio-comment-subscribe@lists.oasis-open.org=0D Unsubscribe: virtio-comment-unsubscribe@lists.oasis-open.org=0D List help: virtio-comment-help@lists.oasis-open.org=0D List archive: https://lists.oasis-open.org/archives/virtio-comment/=0D Feedback License: https://www.oasis-open.org/who/ipr/feedback_license.pdf= =0D List Guidelines: https://www.oasis-open.org/policies-guidelines/mailing-lis= ts=0D Committee: https://www.oasis-open.org/committees/virtio/=0D Join OASIS: https://www.oasis-open.org/join/