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Subject: Re: [virtio-comment] Re: [PATCH v2 0/2] Add VIRTIO_RING_F_LARGE_INDIRECT_DESC
On Tue, Nov 30 2021, Christian Schoenebeck <qemu_oss@crudebyte.com> wrote: > On Dienstag, 30. November 2021 14:48:50 CET Cornelia Huck wrote: >> On Tue, Nov 30 2021, Christian Schoenebeck <qemu_oss@crudebyte.com> wrote: >> > And what about the intended availability of this new virtio_pci_common_cfg >> > field "queue_indirect_size", should it be optional per se, independent of >> > the virtio version or rather a mandatory field in upcoming virtio >> > version? >> We should keep that field depending upon the feature bit IMHO. > > So you are suggesting the existence of "queue_indirect_size" field to be > dependent on feature flag VIRTIO_RING_F_LARGE_INDIRECT_DESC instead of being > dependent on the virtio version. Yes. > Stefan, would that also suit your intended 2nd use case of lowering the max. > descriptor count *below* Queue Size? It would be somewhat different from what > I suggested here in patch 2 [which was min(QueueSize, queue_indirect_size) if > VIRTIO_RING_F_LARGE_INDIRECT_DESC not set], but it could be fine as well. I think that a configurable value for the descriptor count needs to depend on the feature bit as well. > Maybe the name VIRTIO_RING_F_LARGE_INDIRECT_DESC might then a bit misleading > though, because the flag would also be set for forcing small limits. VIRTIO_RING_F_CONFIGURABLE_INDIRECT_DESC ? > > Another issue I just realized: there is also an ambiguity in this v2 what the > maximum descriptor count actually relates to. Should it be > > 1. max. indirect descriptor count per indirect descriptor table > > or > > 2. max. indirect descriptor count per vring slot (i.e. the sum from multiple > indirect descriptor tables within the same message) > > Case 2 applies to QEMU's implementation right now AFAICS. The max. possible > bulk transfer size is lower in case 2 accordingly.
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