linux-kernel - Re: [PATCH] KVM: Use IRQF_ONESHOT for assigned device MSI interrupts

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Message-ID: <4FD1AFD5.70808@siemens.com>
Date:	Fri, 08 Jun 2012 09:55:01 +0200
From:	Jan Kiszka <jan.kiszka@...mens.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
CC:	Thomas Gleixner <tglx@...utronix.de>, Avi Kivity <avi@...hat.com>,
	Alex Williamson <alex.williamson@...hat.com>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"mtosatti@...hat.com" <mtosatti@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"yongjie.ren@...el.com" <yongjie.ren@...el.com>
Subject: Re: [PATCH] KVM: Use IRQF_ONESHOT for assigned device MSI interrupts

On 2012-06-08 09:47, Michael S. Tsirkin wrote:
> On Mon, Jun 04, 2012 at 01:40:28PM +0200, Jan Kiszka wrote:
>> On 2012-06-04 13:21, Thomas Gleixner wrote:
>>> On Sun, 3 Jun 2012, Avi Kivity wrote:
>>>
>>>> On 06/01/2012 09:26 PM, Jan Kiszka wrote:
>>>>>
>>>>>> you suggesting we need a request_edge_threaded_only_irq() API?  Thanks,
>>>>>
>>>>> I'm just wondering if that restriction for threaded IRQs is really
>>>>> necessary for all use cases we have. Threaded MSIs do not appear to me
>>>>> like have to be handled that conservatively, but maybe I'm missing some
>>>>> detail.
>>>>>
>>>>
>>>> btw, I'm hoping we can unthread assigned MSIs.  If the delivery is
>>>> unicast, we can precalculate everything and all the handler has to do is
>>>> set the IRR, KVM_REQ_EVENT, and kick the vcpu.  All of these can be done
>>>> from interrupt context with just RCU locking.
>>>
>>> There is really no need to run MSI/MSI-X interrupts threaded for
>>> KVM. I'm running the patch below for quite some time and it works like
>>> a charm.
>>>
>>> Thanks,
>>>
>>> 	tglx
>>> ----
>>> Index: linux-2.6/virt/kvm/assigned-dev.c
>>> ===================================================================
>>> --- linux-2.6.orig/virt/kvm/assigned-dev.c
>>> +++ linux-2.6/virt/kvm/assigned-dev.c
>>> @@ -105,7 +105,7 @@ static irqreturn_t kvm_assigned_dev_thre
>>>  }
>>>  
>>>  #ifdef __KVM_HAVE_MSI
>>> -static irqreturn_t kvm_assigned_dev_thread_msi(int irq, void *dev_id)
>>> +static irqreturn_t kvm_assigned_dev_msi_handler(int irq, void *dev_id)
>>>  {
>>>  	struct kvm_assigned_dev_kernel *assigned_dev = dev_id;
>>>  
>>> @@ -117,7 +117,7 @@ static irqreturn_t kvm_assigned_dev_thre
>>>  #endif
>>>  
>>>  #ifdef __KVM_HAVE_MSIX
>>> -static irqreturn_t kvm_assigned_dev_thread_msix(int irq, void *dev_id)
>>> +static irqreturn_t kvm_assigned_dev_msix_handler(int irq, void *dev_id)
>>>  {
>>>  	struct kvm_assigned_dev_kernel *assigned_dev = dev_id;
>>>  	int index = find_index_from_host_irq(assigned_dev, irq);
>>> @@ -346,9 +346,8 @@ static int assigned_device_enable_host_m
>>>  	}
>>>  
>>>  	dev->host_irq = dev->dev->irq;
>>> -	if (request_threaded_irq(dev->host_irq, NULL,
>>> -				 kvm_assigned_dev_thread_msi, 0,
>>> -				 dev->irq_name, dev)) {
>>> +	if (request_irq(dev->host_irq, kvm_assigned_dev_msi_handler, 0,
>>> +			dev->irq_name, dev)) {
>>>  		pci_disable_msi(dev->dev);
>>>  		return -EIO;
>>>  	}
>>> @@ -373,9 +372,9 @@ static int assigned_device_enable_host_m
>>>  		return r;
>>>  
>>>  	for (i = 0; i < dev->entries_nr; i++) {
>>> -		r = request_threaded_irq(dev->host_msix_entries[i].vector,
>>> -					 NULL, kvm_assigned_dev_thread_msix,
>>> -					 0, dev->irq_name, dev);
>>> +		r = request_irq(dev->host_msix_entries[i].vector,
>>> +				kvm_assigned_dev_msix_handler, 0,
>>> +				dev->irq_name, dev);
>>>  		if (r)
>>>  			goto err;
>>>  	}
>>
>> This may work in practice but has two conceptual problems:
>>  - we do not want to run a potential broadcast to all VCPUs to run in
>>    a host IRQ handler
>>  - crazy user space could have configured the route to end up in the
>>    PIC or IOAPIC, and both are not hard-IRQ safe (this should probably
>>    be caught on setup)
>>
>> So this shortcut requires some checks before being applied to a specific
>> MSI/MSI-X vector.
> 
> I did this in the past:
> https://lkml.org/lkml/2012/1/18/287
> 
> Have no hw to test this atm but if there are any takers
> wanting to play with it I can update and post.

Just add check that allow only unicasts, and this should be fine.

Jan

-- 
Siemens AG, Corporate Technology, CT T DE IT 1
Corporate Competence Center Embedded Linux
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