On Fri, Oct 20, 2023 at 04:16:16PM +0100, Andrew Cooper wrote: > On 20/10/2023 3:50 pm, Durrant, Paul wrote: > > On 20/10/2023 14:37, David Woodhouse wrote: > > [snip] > >>> > >>> [0] > >>> https://elixir.bootlin.com/linux/latest/source/drivers/tty/hvc/hvc_xen.c#L258 > >> > >> I'm not convinced I believe what the comment says there about evtchn 0 > >> being theoretically valid. I don't believe zero is a valid evtchn#, is > >> it? > > > > gfn 0 might be valid, but I'm also pretty sure evtchn 0 is not valid. > > GFN 0 very much is valid. > > evtchn 0 OTOH is explicitly not valid. From evtchn_init(): > > evtchn_from_port(d, 0)->state = ECS_RESERVED; > > > However, the fields being 0 doesn't mean not available. That's the > signal to saying "not connected yet", because that's what dom0 gets > before xenconsoled starts up.
Someone asked me the same a while back, and IIRC we don't state anywhere in the public headers that event channel 0 is reserved, however that has always? been part of the implementation. If we intend this to be reliable, we should add a define to the public headers in order to signal that 0 will always be reserved. Roger.
