Signed-off-by: Rusty Russell <> --- content.tex
14 +++++++++++--- 1 file changed, 11 insertions(+), 3 deletions(-) diff --git a/content.tex b/content.tex index 1decb0a..966b293 100644 --- a/content.tex +++ b/content.tex @@ -290,8 +290,7 @@ endian of the guest, not little-endian as specified by this standard. It is assumed that the host is already aware of the guest endian.
subsection{Message Framing}label{sec:Basic Facilities of a Virtio Device / Virtqueues / Message Framing} -The device MUST NOT make assumptions about the particular arrangement -of descriptors: the message framing is +The framing of messages with descriptors is independent of the contents of the buffers. For example, a network transmit buffer consists of a 12 byte header followed by the network packet. This could be most simply placed in the descriptor table as a @@ -300,13 +299,22 @@ but it could also consist of a single 1526 byte output descriptor in the case where the header and packet are adjacent, or even three or more descriptors (possibly with loss of efficiency in that case).
-Note that, some implementations may have large-but-reasonable +Note that, some device implementations have large-but-reasonable restrictions on total descriptor size (such as based on IOV_MAX in the host OS). This has not been a problem in practice: little sympathy will be given to drivers which create unreasonably-sized descriptors such as by dividing a network packet into 1500 single-byte descriptors!
+devicenormative{Basic Facilities of a Virtio Device / Message Framing} +The device MUST NOT make assumptions about the particular arrangement +of descriptors. The device MAY have a reasonable limit of descriptors +it will allow in a chain. + +drivernormative{Basic Facilities of a Virtio Device / Message Framing} +The driver SHOULD NOT use an excessive number of descriptors to +describe a buffer. + subsubsection{Legacy Interface: Message Framing}label{sec:Basic Facilities of a Virtio Device / Virtqueues / Message Framing / Legacy Interface: Message Framing}
Regrettably, initial driver implementations used simple layouts, and -- 1.8.3.2
