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Re: [PATCH] ns16550c: avoid crash in ns16550_endboot in PV shim mode
- To: Roger Pau Monné <roger.pau@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Mon, 23 Oct 2023 10:05:48 +0200
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- Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Delivery-date: Mon, 23 Oct 2023 08:05:56 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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.
Jan
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