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Subject: Re: [PATCH v2 1/4] Add VIRTIO_RING_F_INDIRECT_SIZE
On Donnerstag, 10. März 2022 15:57:28 CET Stefan Hajnoczi wrote: > On Mon, Feb 21, 2022 at 05:25:00PM +0100, Christian Schoenebeck wrote: > > @@ -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 > > + 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 > s/bus/transport/ > > Otherwise: > Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com> I'll change that in v3. Thanks! Best regards, Christian Schoenebeck
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