linux-kernel - Re: [PATCH] drivers/xen: Improve the late XenStore init protocol

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Date: Fri, 17 May 2024 09:17:43 +0800
From: Henry Wang <xin.wang2@....com>
To: Stefano Stabellini <sstabellini@...nel.org>
CC: <xen-devel@...ts.xenproject.org>, <linux-kernel@...r.kernel.org>, "Juergen
 Gross" <jgross@...e.com>, Oleksandr Tyshchenko
	<oleksandr_tyshchenko@...m.com>, Michal Orzel <michal.orzel@....com>
Subject: Re: [PATCH] drivers/xen: Improve the late XenStore init protocol

Hi Stefano,

On 5/17/2024 8:52 AM, Stefano Stabellini wrote:
> On Thu, 16 May 2024, Henry Wang wrote:
>>>>    enum xenstore_init xen_store_domain_type;
>>>>    EXPORT_SYMBOL_GPL(xen_store_domain_type);
>>>>    @@ -751,9 +755,10 @@ static void xenbus_probe(void)
>>>>    {
>>>>    	xenstored_ready = 1;
>>>>    -	if (!xen_store_interface) {
>>>> -		xen_store_interface = memremap(xen_store_gfn <<
>>>> XEN_PAGE_SHIFT,
>>>> -					       XEN_PAGE_SIZE, MEMREMAP_WB);
>>>> +	if (!xen_store_interface || XS_INTERFACE_READY) {
>>>> +		if (!xen_store_interface)
>>> These two nested if's don't make sense to me. If XS_INTERFACE_READY
>>> succeeds, it means that  ((xen_store_interface != NULL) &&
>>> (xen_store_interface->connection == XENSTORE_CONNECTED)).
>>>
>>> So it is not possible that xen_store_interface == NULL immediately
>>> after. Right?
>> I think this is because we want to free the irq for the late init case,
>> otherwise the init-dom0less will fail. For the xenstore PFN allocated case,
>> the connection is already set to CONNECTED when we execute init-dom0less. But
>> I agree with you, would below diff makes more sense to you?
>>
>> diff --git a/drivers/xen/xenbus/xenbus_probe.c
>> b/drivers/xen/xenbus/xenbus_probe.c
>> index 8aec0ed1d047..b8005b651a29 100644
>> --- a/drivers/xen/xenbus/xenbus_probe.c
>> +++ b/drivers/xen/xenbus/xenbus_probe.c
>> @@ -76,6 +76,8 @@ EXPORT_SYMBOL_GPL(xen_store_interface);
>>          ((xen_store_interface != NULL) && \
>>           (xen_store_interface->connection == XENSTORE_CONNECTED))
>>
>> +static bool xs_late_init = false;
>> +
>>   enum xenstore_init xen_store_domain_type;
>>   EXPORT_SYMBOL_GPL(xen_store_domain_type);
>>
>> @@ -755,7 +757,7 @@ static void xenbus_probe(void)
>>   {
>>          xenstored_ready = 1;
>>
>> -       if (!xen_store_interface || XS_INTERFACE_READY) {
>> +       if (xs_late_init) {
>>                  if (!xen_store_interface)
>>                          xen_store_interface = memremap(xen_store_gfn <<
>
> I would just remove the outer 'if' and do this:
>
>
> 	if (!xen_store_interface)
> 		xen_store_interface = memremap(xen_store_gfn << XEN_PAGE_SHIFT,
> 				XEN_PAGE_SIZE, MEMREMAP_WB);
> 	/*
> 	 * Now it is safe to free the IRQ used for xenstore late
> 	 * initialization. No need to unbind: it is about to be
> 	 * bound again from xb_init_comms. Note that calling
> 	 * unbind_from_irqhandler now would result in xen_evtchn_close()
> 	 * being called and the event channel not being enabled again
> 	 * afterwards, resulting in missed event notifications.
> 	 */
> 	if (xs_init_irq > 0)
> 		free_irq(xs_init_irq, &xb_waitq);
>
>
> I think this should work fine in all cases.

Thanks. I followed your suggestion in v2.

>   I am unsure if
> xs_init_irq==0 is possible valid value for xs_init_irq. If it is not,
> then we are fine. If 0 is a possible valid irq number, then we should
> initialize xs_init_irq to -1, and here check for xs_init_irq >= 0.

Yeah the xs_init_irq==0 is a valid value. I followed your latter comment 
to init it to -1 and check it >=0.

Kind regards,
Henry