Re: FIFO-based event channel ABI design (draft B)

["Jan Beulich" <JBeulich@suse.com>]

>>> On 15.02.13 at 15:32, David Vrabel <david.vrabel@citrix.com> wrote:
> ### Control Block
> 
> ![\label{fig_control-block}Control Block](control-block.pdf)
> 
> The READY field contains a bit for each priority's queue.  A set bit
> indicates that there are events pending on that queue.  A queue's
> ready bit is set by Xen when an event is placed on an empty queue and
> cleared by the guest when it empties the queue.
> 
> There are N HEAD and TAIL indexes, one for each priority.
> 
> The HEAD index is the first event in the queue or zero if the queue is
> empty.  HEAD is set by the guest as it consumes events and only set by
> Xen when adding an event to an empty queue.
> 
> The TAIL index is the last event in the queue.  It is undefined if the
> queue is empty.  TAIL is only set by Xen.

One thing I continue to wonder is why tail it part of the control
block at all - from all I can tell, it's not consumed by the guest (end
of list being indicated by a zero LINK field), and hence is an
implementation detail of the hypervisor. And given that the
hypervisor reads that field, not exposing the field also avoids the
need to validate is each time it gets read from the control block.

Additionally, I think the number of significant bits of the LINK field
should actually be obtained from the hypervisor, rather than being
baked at 17 (for the time being). This again is an implementation
detail (and the value could easily be command line controllable).

Jan