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Re: [PATCH] ns16550c: avoid crash in ns16550_endboot in PV shim mode
- To: Jan Beulich <jbeulich@xxxxxxxx>
- From: Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Date: Mon, 23 Oct 2023 10:17:15 +0200
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- Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Delivery-date: Mon, 23 Oct 2023 08:17:29 +0000
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On Mon, Oct 23, 2023 at 10:05:48AM +0200, Jan Beulich wrote:
> On 23.10.2023 09:52, Roger Pau Monné wrote:
> > 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.
>
> I agree a comment should have been there; it's not clear to me what
> useful #define we could add.
`EVTCHN_PORT_INVALID 0` or some such, but a comment would also be
fine, the point is to be part of the public interface.
Thanks, Roger.
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