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Subject: Re: [virtio] [PATCH] virtqueue: flexible layout, size, alignment
"Michael S. Tsirkin" <mst@redhat.com> writes: > Transports can now lay out available/used/descriptor > regions in a flexible way. > This is useful for embedded systems to save memory, > and for large systems to reduce the need for > physically-contigious memory. > > This does not add a way to actually program this > in any of the transports, so it's not useful by > itself, a follow-up patch with add a way to > program this for PCI. > > Signed-off-by: Michael S. Tsirkin <mst@redhat.com> > --- > virtio-v1.0-wd01-part1-specification.txt | 52 ++++++++++++++++++++++++++++---- > 1 file changed, 46 insertions(+), 6 deletions(-) > > diff --git a/virtio-v1.0-wd01-part1-specification.txt b/virtio-v1.0-wd01-part1-specification.txt > index fcd9fd7..9a40973 100644 > --- a/virtio-v1.0-wd01-part1-specification.txt > +++ b/virtio-v1.0-wd01-part1-specification.txt > @@ -223,6 +223,45 @@ transmit and one for receive. Each queue has a 16-bit queue size > parameter, which sets the number of entries and implies the total size > of the queue. > > +Each virtqueue consists of three parts: > + > + Descriptor Table > + Available Ring > + Used Ring > + > +where each part is physically-contiguous in guest memory, > +and has different alignment requirements. > + > +The Queue Size field controls the total number of bytes > +required for each part of the virtqueue. > + > +The memory aligment and size requirements, in bytes, of each part of the > +virtqueue are summarized in the following table (qsz is the Queue Size field): > + > ++------------+---------------------------------+ > +| Virtqueue Part | Alignment | Size | > ++------------+---------------------------------+ > ++------------+---------------------------------+ > +| Descriptor Table | 16 | 16 * qsz | Alignment 16: isn't that overkill? > ++------------+---------------------------------+ > +| Available Ring | 2 | 6 + 2 * qsz | > ++------------+---------------------------------+ > +| Used Ring | 4 | 6 + 4 * qsz | > ++------------+---------------------------------+ > + > +When the driver wants to send a buffer to the device, it fills in > +a slot in the descriptor table (or chains several together), and > +writes the descriptor index into the available ring. It then > +notifies the device. When the device has finished a buffer, it > +writes the descriptor into the used ring, and sends an interrupt. > + > + > +100.100.4.1. Legacy Interfaces: A Note on Virtqueue Layout > +-------------------------------------- > + > +For Legacy Interfaces, several additional > +restrictions are placed on the virtqueue layout: > + > Each virtqueue occupies two or more physically-contiguous pages > (usually defined as 4096 bytes, but depending on the transport) > and consists of three parts: > @@ -241,9 +280,8 @@ required for the virtqueue according to the following formula: > + ALIGN(sizeof(u16)*3 + sizeof(struct vring_used_elem)*qsz); > } > > -This currently wastes some space with padding, but also allows future > -extensions such as the VIRTIO_RING_F_EVENT_IDX extension. The > -virtqueue layout structure looks like this: > +This wastes some space with padding. > +The legacy virtqueue layout structure therefore looks like this: > > struct vring { > // The actual descriptors (16 bytes each) > @@ -814,9 +852,11 @@ This is done as follows, for each virtqueue a device has: > always a power of 2. This controls how big the virtqueue is > (see "2.1.4. Virtqueues"). If this field is 0, the virtqueue does not exist. > > -3. Allocate and zero virtqueue in contiguous physical memory, on > - a 4096 byte alignment. Write the physical address, divided by > - 4096 to the Queue Address field.[6] > +3. Optionally, select a smaller virtqueue size and write it in the Queue Size > + field. > + > +3. Allocate and zero Descriptor Table, Available and Used rings for the > + virtqueue in contiguous physical memory. > > 4. Optionally, if MSI-X capability is present and enabled on the > device, select a vector to use to request interrupts triggered We should move most of this out to the PCI-specific section. For example, mmio has a QueueNumMax field instead, and ccw doesn't seem to have a way of negotiating. Cheers, Rusty.
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