On 04/13/2012 09:50 AM, Stefan Hajnoczi wrote:
The virtio specification says: "The descriptors used for a buffer should not effect the semantics of the message, except for the total length of the buffer" and "In particular, no implementation should use the descriptor boundaries to determine the size of any header in a request"
This was the noble intention but all of the implementations actually rely on boundary sizes.
Both QEMU and lguest rely on boundary sizes. We've removed some of it in virtio-net in QEMU but it still looks like it's there for virtio-blk.
kvm tool also makes this assumption.
Why should descriptor layout not be specified? It seems that implementing arbitrary descriptor layout support (e.g. 1-byte descriptors) requires more code and makes input validation harder. Why bother with the flexibility of unspecified descriptor layouts? As long as the layout is specified clearly it makes everyone's lives easier to use a strict descriptor layout.
I hate to just change the spec here but I don't see a better option. Regards, Anthony Liguori
The only reason I can think of is that virtio should work over transports that do not have the concept of "descriptors" (non-vring transports like pipes or streams). Stefan
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