linux-kernel - Re: [PATCH v2] xen: Use evtchn_type

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Date:   Tue, 10 Mar 2020 07:21:58 +0100
From:   Jürgen Groß <jgross@...e.com>
To:     Yan Yankovskyi <yyankovskyi@...il.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>
Cc:     Jan Beulich <jbeulich@...e.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] xen: Use evtchn_type_t as a type for event channels

On 08.03.20 14:19, Yan Yankovskyi wrote:
> On Sat, Mar 07, 2020 at 02:41:44PM -0500, Boris Ostrovsky wrote:
>>
>>
>> On 3/7/20 8:43 AM, Yan Yankovskyi wrote:
>>> Make event channel functions pass event channel port using
>>> evtchn_port_t type. It eliminates signed <-> unsigned conversion.
>>>
>>
>>
>>>   static int find_virq(unsigned int virq, unsigned int cpu)
>>>   {
>>>   	struct evtchn_status status;
>>> -	int port, rc = -ENOENT;
>>> +	evtchn_port_t port;
>>> +	int rc = -ENOENT;
>>>   
>>>   	memset(&status, 0, sizeof(status));
>>>   	for (port = 0; port < xen_evtchn_max_channels(); port++) {
>>> @@ -962,7 +963,8 @@ EXPORT_SYMBOL_GPL(xen_evtchn_nr_channels);
>>>   int bind_virq_to_irq(unsigned int virq, unsigned int cpu, bool percpu)
>>>   {
>>>   	struct evtchn_bind_virq bind_virq;
>>> -	int evtchn, irq, ret;
>>> +	evtchn_port_t evtchn = xen_evtchn_max_channels();
>>> +	int irq, ret;
>>>   
>>>   	mutex_lock(&irq_mapping_update_lock);
>>>   
>>> @@ -990,7 +992,6 @@ int bind_virq_to_irq(unsigned int virq, unsigned int cpu, bool percpu)
>>>   			if (ret == -EEXIST)
>>>   				ret = find_virq(virq, cpu);
>>>   			BUG_ON(ret < 0);
>>> -			evtchn = ret;
>>
>>
>> This looks suspicious. What would you be passing to
>> xen_irq_info_virq_setup() below?
> 
> Right, this line should be preserved.
> 
>> I also think that, given that this patch is trying to get types in
>> order, find_virq() will need more changes: it is supposed to return
>> evtchn_port_t. But then it also wants to return a (signed) error.
>   
> As we don't care which error we got during find_virq call, we can just
> return 0 in case of error, and port number otherwise. Port 0 is never
> valid, so this approach can work for the other functions as well.
> On the other hand, passing port using pointer and returning actual
> error message, as it's done in xenbus_alloc_evtchn(), sounds like a
> better approach overall. What do you think?

You can use the same approach as Xen tools do and define something like:

typedef int evtchn_port_or_error_t;


Juergen