Date: Wed, 7 Feb 2024 15:30:15 -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 3:19 PM, Alex Williamson wrote:
> On Tue, 6 Feb 2024 14:22:04 -0800
> Reinette Chatre <reinette.chatre@...el.com> wrote:
>> 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:
>>>> 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.
> 
> Ah yes, I see the dilemma now.  INTx always checked start/count were
> valid but MSI/X plowed through regardless, and with this series we've
> standardized the loop around the MSI/X flow.
> 
> Tricky, but probably doesn't really matter.  Unless we break someone.
> 
> I can ignore that INTx can be masked and signaling a masked vector
> doesn't do anything, but signaling an unconfigured vector feels like an
> error condition and trying to create verbiage in the uAPI header to
> weasel out of that error and unconditionally return success makes me
> cringe.
> 
> What if we did this:
> 
>         uint8_t *bools = data;
> 	...
>         for (i = start; i < start + count; i++) {
>                 if ((flags & VFIO_IRQ_SET_DATA_NONE) ||
>                     ((flags & VFIO_IRQ_SET_DATA_BOOL) && bools[i - start])) {
>                         ctx = vfio_irq_ctx_get(vdev, i);
>                         if (!ctx || !ctx->trigger)
>                                 return -EINVAL;
>                         intr_ops[index].send_eventfd(vdev, ctx);
>                 }
>         }
> 

This looks good. Thank you very much. Will do.

I studied the code more and have one more observation related to this portion
of the flow:
>From what I can tell this change makes the INTx code more robust. If I
understand current implementation correctly it seems possible to enable
INTx but not have interrupt allocated. In this case the interrupt context
(ctx) will exist but ctx->trigger will be NULL. Current
vfio_pci_set_intx_trigger()->vfio_send_intx_eventfd() only checks if
ctx is valid. It looks like it may call eventfd_signal(NULL) where
pointer is dereferenced.

If this is correct then I think a separate fix that can easily be
backported may be needed. Something like:

diff --git a/drivers/vfio/pci/vfio_pci_intrs.c b/drivers/vfio/pci/vfio_pci_intrs.c
index 237beac83809..17ec46d8ab29 100644
--- a/drivers/vfio/pci/vfio_pci_intrs.c
+++ b/drivers/vfio/pci/vfio_pci_intrs.c
@@ -92,7 +92,7 @@ static void vfio_send_intx_eventfd(void *opaque, void *unused)
 		struct vfio_pci_irq_ctx *ctx;
 
 		ctx = vfio_irq_ctx_get(vdev, 0);
-		if (WARN_ON_ONCE(!ctx))
+		if (WARN_ON_ONCE(!ctx || !ctx->trigger))
 			return;
 		eventfd_signal(ctx->trigger);
 	}

> And we note the behavior change for MSI/X in the commit log and if
> someone shouts that we broke them, we can make that an -errno or
> continue based on is_intx().  Sound ok?  Thanks,

I'll be sure to highlight the impact on MSI/MSI-x. Please do expect this
in the final patch "vfio/pci: Remove duplicate interrupt management flow"
though since that is where the different flows are merged.

I am not familiar with how all user space interacts with this flow and if/how
this may break things. I did look at Qemu code and I was not able to find
where it intentionally triggers MSI/MSI-x interrupts, I could only find it
for INTx.

If this does break things I would like to also consider moving the
different behavior into the interrupt type's respective send_eventfd()
callback instead of adding interrupt type specific code (like
is_intx()) into the shared flow.

Thank you.

Reinette

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