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Re: [PATCH] ns16550c: avoid crash in ns16550_endboot in PV shim mode
- To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- From: Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Date: Mon, 23 Oct 2023 09:52:25 +0200
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- Delivery-date: Mon, 23 Oct 2023 07:52:41 +0000
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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.
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