linux-kernel - Re: [PATCH 15/17] vfio/pci: Let enable and disable of interrupt types use same signature

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Date: Tue, 6 Feb 2024 14:22:04 -0800
From: Reinette Chatre <reinette.chatre@...el.com>
To: Alex Williamson <alex.williamson@...hat.com>
CC: <jgg@...dia.com>, <yishaih@...dia.com>,
	<shameerali.kolothum.thodi@...wei.com>, <kevin.tian@...el.com>,
	<kvm@...r.kernel.org>, <dave.jiang@...el.com>, <ashok.raj@...el.com>,
	<linux-kernel@...r.kernel.org>, <patches@...ts.linux.dev>
Subject: Re: [PATCH 15/17] vfio/pci: Let enable and disable of interrupt types
 use same signature

Hi Alex,

On 2/6/2024 2:03 PM, Alex Williamson wrote:
> On Tue, 6 Feb 2024 13:46:37 -0800
> Reinette Chatre <reinette.chatre@...el.com> wrote:
> 
>> Hi Alex,
>>
>> On 2/5/2024 2:35 PM, Alex Williamson wrote:
>>> On Thu,  1 Feb 2024 20:57:09 -0800
>>> Reinette Chatre <reinette.chatre@...el.com> wrote:  
>>
>> ..
>>
>>>> @@ -715,13 +724,13 @@ static int vfio_pci_set_intx_trigger(struct vfio_pci_core_device *vdev,
>>>>  		if (is_intx(vdev))
>>>>  			return vfio_irq_set_block(vdev, start, count, fds, index);
>>>>  
>>>> -		ret = vfio_intx_enable(vdev);
>>>> +		ret = vfio_intx_enable(vdev, start, count, index);  
>>>
>>> Please trace what happens when a user calls SET_IRQS to setup a trigger
>>> eventfd with start = 0, count = 1, followed by any other combination of
>>> start and count values once is_intx() is true.  vfio_intx_enable()
>>> cannot be the only place we bounds check the user, all of the INTx
>>> callbacks should be an error or nop if vector != 0.  Thanks,
>>>   
>>
>> Thank you very much for catching this. I plan to add the vector
>> check to the device_name() and request_interrupt() callbacks. I do
>> not think it is necessary to add the vector check to disable() since
>> it does not operate on a range and from what I can tell it depends on
>> a successful enable() that already contains the vector check. Similar,
>> free_interrupt() requires a successful request_interrupt() (that will
>> have vector check in next version).
>> send_eventfd() requires a valid interrupt context that is only
>> possible if enable() or request_interrupt() succeeded.
> 
> Sounds reasonable.
> 
>> If user space creates an eventfd with start = 0 and count = 1
>> and then attempts to trigger the eventfd using another combination then
>> the changes in this series will result in a nop while the current
>> implementation will result in -EINVAL. Is this acceptable?
> 
> I think by nop, you mean the ioctl returns success.  Was the call a
> success?  Thanks,

Yes, I mean the ioctl returns success without taking any
action (nop).

It is not obvious to me how to interpret "success" because from what I
understand current INTx and MSI/MSI-x are behaving differently when
considering this flow. If I understand correctly, INTx will return
an error if user space attempts to trigger an eventfd that has not
been set up while MSI and MSI-x will return 0.

I can restore existing INTx behavior by adding more logic and a return
code to the send_eventfd() callback so that the different interrupt types
can maintain their existing behavior.

Reinette