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Subject: Re: [PATCH v3 1/4] Add VIRTIO_RING_F_INDIRECT_SIZE
On Donnerstag, 17. März 2022 14:40:27 CET Cornelia Huck wrote: > On Wed, Mar 16 2022, 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. > > > > 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 > > (...) > > > @@ -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 > "supported by the device", or maybe "it supports"? Article "the" is missing here, yes. Both "the device" or "it" would be fine. > > +configuration. It MUST allow the driver to set a lower value. > > Maybe "It MUST allow the driver to specify a lower maximum size." ? That exact phrase was actually suggested by you (2021-12-14 18:20). I have no strong opinion on that. I find the existing "to set a lower value" clear enough and a less complicated wording though. > > + > > > > \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. > > + > > + With this feature enabled, the Queue Size only limits the maximum > > amount > > s/enabled/negotiated/ ? Also no strong opinion on that. "negotiated" makes it a bit more clear, yes. > > + 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. > > No 'MUST' in non-normative sections, please. Maybe make that > > "and therefore the device and the driver additionally need to negotiate > the transport specific Queue Indirect Size value if > VIRTIO_RING_F_INDIRECT_SIZE was negotiated in order to determine the > actual amount of maximum indirect descriptors supported by both." Yes, that still slipped through, sorry. That change looks fine to me. > I'm not sure whether we would actually need some normative statements in > the sections below, but probably not. Like what would you potentially miss here? > > + > > > > \end{description} > > > > \drivernormative{\section}{Reserved Feature Bits}{Reserved Feature Bits}
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