Re: [virtio-comment] [PATCH v3 1/4] Add VIRTIO_RING_F_INDIRECT_SIZE

[Christian Schoenebeck <qemu_oss@crudebyte.com>]

On Samstag, 19. März 2022 10:33:49 CET Michael S. Tsirkin wrote:
> On Wed, Mar 16, 2022, 15:47 Christian Schoenebeck <qemu_oss@crudebyte.com>
> 
> wrote:
> > This new feature flag allows to decouple the maximum amount of
> > descriptors in indirect descriptor tables from the Queue Size.
> 
> if we are extending these limits, I suggest reusing the feature flag to
> also add a limit on total s/g list size. making it separate from queue size
> was requested a while ago.

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?

And are you suggesting this should become part of this series already?

> > The new term "Queue Indirect Size" is introduced for this purpose,
> > which is a transport specific configuration whose negotiation is
> > further specified for each transport with subsequent patches.
> > 
> > Fixes: https://github.com/oasis-tcs/virtio-spec/issues/122
> > Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
> > Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
> > ---
> > 
> >  content.tex     | 32 ++++++++++++++++++++++++++++++--
> >  packed-ring.tex |  2 +-
> >  split-ring.tex  |  8 ++++++--
> >  3 files changed, 37 insertions(+), 5 deletions(-)
> > 
> > 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
> > 
> >  \begin{description}
> >  \item[0 to 23, and 50 to 127] Feature bits for the specific device type
> > 
> > -\item[24 to 40] Feature bits reserved for extensions to the queue and
> > +\item[24 to 41] Feature bits reserved for extensions to the queue and
> > 
> >    feature negotiation mechanisms
> > 
> > -\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.
> > 
> >  \end{description}
> >  
> >  \begin{note}
> > 
> > @@ -1051,6 +1051,10 @@ \subsubsection{Common configuration structure
> > layout}\label{sec:Virtio Transport
> > 
> >  present either a value of 0 or a power of 2 in
> >  \field{queue_size}.
> > 
> > +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.
> > +
> > 
> >  \drivernormative{\paragraph}{Common configuration structure
> > 
> > layout}{Virtio Transport Options / Virtio Over PCI Bus / PCI Device Layout
> > / Common configuration structure layout}
> > 
> >  The driver MUST NOT write to \field{device_feature}, \field{num_queues},
> > 
> > \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}
> > 
> >    that the driver can reset a queue individually.
> >    See \ref{sec:Basic Facilities of a Virtio Device / Virtqueues /
> > 
> > Virtqueue Reset}.
> > 
> > +  \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.
> > +
> > +  Without this feature, the Queue Size limits the length of the
> > descriptor
> > +  chain, including indirect descriptor tables as in \ref{sec:Basic
> > Facilities of
> > +  a Virtio Device / Virtqueues / The Virtqueue Descriptor Table /
> > Indirect
> > +  Descriptors}, i.e. both the maximum amount of slots in the vring and
> > the
> > +  actual bulk data size transmitted per vring slot.
> 
> spect does not call these  slots elsewhere.

Yes, I intentionally used "vring slot" instead of "buffer" as I find the 
latter too vague in this context. A "buffer" can be a memory segment, a set of 
memory segments and what not. "vring slot" OTOH makes it clear that it is 
about exactly one, atomic pointer (hence with fixed size) in a Ring Buffer, as 
depicted in the ASCII illustration here:

https://github.com/oasis-tcs/virtio-spec/issues/122

The maximum amount of vring slots is therefore the maximum amount of messages 
that can be emplaced into a Ring Buffer, independent of any "bulk data buffer 
size".

> 
> 
> +
> 
> > +  With this feature enabled, the Queue Size only limits the maximum
> > amount
> > +  of slots in the vring, but does not limit the actual bulk data 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 complicated
> > synchronization
> > +  mechanisms if the device only supports a very small amount of 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 MUST
> > 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.
> 
> still not sure what exactly is the value. e.g. in a buffer including
> indirect and direct descriptors.
> 
> +
> 
> >  \end{description}
> >  
> >  \drivernormative{\section}{Reserved Feature Bits}{Reserved Feature Bits}
> > 
> > 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}
> > 
> >  The device limits the number of descriptors in a list through a
> >  transport-specific and/or device-specific value. If not limited,
> >  the maximum number of descriptors in a list is the virt queue
> > 
> > -size.
> > +size unless the VIRTIO_RING_F_INDIRECT_SIZE feature has been negotiated.
> > 
> >  \subsection{Next Flag: Descriptor Chaining}
> >  \label{sec:Packed Virtqueues / Next Flag: Descriptor Chaining}
> > 
> > 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
> > 
> >  set the VIRTQ_DESC_F_INDIRECT flag within an indirect descriptor (ie.
> >  only
> >  one table per descriptor).
> > 
> > -A driver MUST NOT create a descriptor chain longer than the Queue Size of
> > -the device.
> 
> +If VIRTIO_RING_F_INDIRECT_SIZE has not been negotiated, the driver MUST
> 
> > +NOT create a descriptor chain longer than the Queue Size of the 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.
> 
> it is not negotiated is it?

What makes you think it is not negotiated?

> 
> >  A driver MUST NOT set both VIRTQ_DESC_F_INDIRECT and VIRTQ_DESC_F_NEXT
> >  in \field{flags}.
> > 
> > --
> > 2.30.2
> > 
> > 
> > 
> > 
> > 
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