Re: [PATCH v6 2/7] xen/events: don't allow binding a global virq from any domain

[Jan Beulich <jbeulich@xxxxxxxx>]

On 08.01.2025 10:02, Jürgen Groß wrote:
> On 07.01.25 17:38, Jan Beulich wrote:
>> On 07.01.2025 17:07, Jürgen Groß wrote:
>>> On 07.01.25 16:34, Jan Beulich wrote:
>>>> On 07.01.2025 11:17, Juergen Gross wrote:
>>>>> @@ -479,8 +486,13 @@ int evtchn_bind_virq(evtchn_bind_virq_t *bind, 
>>>>> evtchn_port_t port)
>>>>>        */
>>>>>        virq = array_index_nospec(virq, ARRAY_SIZE(v->virq_to_evtchn));
>>>>>    
>>>>> -    if ( virq_is_global(virq) && (vcpu != 0) )
>>>>> -        return -EINVAL;
>>>>> +    if ( virq_is_global(virq) )
>>>>> +    {
>>>>> +        if ( get_global_virq_handler(virq) != d )
>>>>> +            return -EBUSY;
>>>>
>>>> Hmm. While this eliminates the problem for the common, race free case,
>>>> the handler changing right after the check would still mean the bind
>>>> would succeed.
>>>
>>> Are you fine with me adding a paragraph to the commit message saying
>>> that a future patch will handle this case?
>>>
>>> This future patch is patch 4 of the series, which will need to be
>>> modified to check the handling domain inside the event_lock.
>>
>> I think this would be okay, so long as patches 2...4 are then also all
>> committed together.
>>
>>>> Plus this way you're breaking a case that afaict has been working so
>>>> far: The bind happening before the setting of the handler. With a lot
>>>> of unrelated if-s and when-s this could e.g. be of interest when
>>>> considering a re-startable Xenstore domain. The one to take over could
>>>> start first, obtain state from the original one while that's still
>>>> active, and be nominated the handler of the global vIRQ only in the
>>>> last moment.
>>>
>>> This is a racy situation, too. If the old domain receives the virq after
>>> sending the state, this would need to be handled by transferring the virq
>>> information to the new domain, which can result in a never ending story.
>>>
>>> This is the reason why the domain state bitmap is reset to contain all
>>> existing domains to be flagged as "changed", as otherwise a change might
>>> get lost.
>>>
>>> I'd rather be able to handle today's use cases in a sane way than to try
>>> handling any weird future use cases which we don't know yet.
>>>
>>> I think today's behavior is more or less insane and the new behavior is
>>> much easier to understand and more intuitive.
>>
>> Hmm, I'd like to leave this then for input by other maintainers.
> 
> Just one additional remark to your re-startable xenstore domain scenario
> above:
> 
> It wouldn't be possible today to do the same with a xenstore daemon in
> e.g. dom0, as binding the virq another time from within the same domain
> would be rejected by the hypervisor. In the xenstore domain case you'd
> either need the old domain to ask dom0 to change the handler (so much
> about less communication needed),

Not quite. There needs to be an indication anyway of info transfer being
complete. That'll be where Dom0 would then (also) put in place the new
handler. The vIRQ first arriving in the new XS domain could then serve
as an indication that it is now in charge of the system; I didn't check
whether a courtesy one would be sent right away, or whether such sending
might need adding. (Plus anyway - XS is only an example here.)

Jan

> or you'd need to give the xenstore domain
> the right to do the handler change itself, requiring to use flask or to
> modify the dummy XSM rights of the xenstore domain.
> 
> 
> Juergen