linux-kernel - Re: [PATCH V4 10/11] vfio/pci: Support dynamic MSI-X

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Date:   Fri, 5 May 2023 10:21:22 -0700
From:   Reinette Chatre <reinette.chatre@...el.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>,
        "jgg@...dia.com" <jgg@...dia.com>,
        "yishaih@...dia.com" <yishaih@...dia.com>,
        "shameerali.kolothum.thodi@...wei.com" 
        <shameerali.kolothum.thodi@...wei.com>,
        "alex.williamson@...hat.com" <alex.williamson@...hat.com>
CC:     "tglx@...utronix.de" <tglx@...utronix.de>,
        "darwi@...utronix.de" <darwi@...utronix.de>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        "Liu, Jing2" <jing2.liu@...el.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "Yu, Fenghua" <fenghua.yu@...el.com>,
        "tom.zanussi@...ux.intel.com" <tom.zanussi@...ux.intel.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH V4 10/11] vfio/pci: Support dynamic MSI-X

Hi Kevin,

On 5/5/2023 1:10 AM, Tian, Kevin wrote:
>> From: Chatre, Reinette <reinette.chatre@...el.com>
>> Sent: Saturday, April 29, 2023 2:35 AM
>> On 4/27/2023 11:50 PM, Tian, Kevin wrote:
>>>> From: Chatre, Reinette <reinette.chatre@...el.com>
>>>> Sent: Friday, April 28, 2023 1:36 AM

...

>>>> +/*
>>>> + * Return Linux IRQ number of an MSI or MSI-X device interrupt vector.
>>>> + * If a Linux IRQ number is not available then a new interrupt will be
>>>> + * allocated if dynamic MSI-X is supported.
>>>> + */
>>>> +static int vfio_msi_alloc_irq(struct vfio_pci_core_device *vdev,
>>>> +			      unsigned int vector, bool msix)
>>>> +{
>>>> +	struct pci_dev *pdev = vdev->pdev;
>>>> +	struct msi_map map;
>>>> +	int irq;
>>>> +	u16 cmd;
>>>> +
>>>> +	irq = pci_irq_vector(pdev, vector);
>>>> +	if (irq > 0 || !msix || !vdev->has_dyn_msix)
>>>> +		return irq;
>>>
>>> if (irq >= 0 || ...)
>>>
>>
>> I am not sure about this request because pci_irq_vector() cannot return 0.
>> The Linux interrupt number will be > 0 on success. 0 means "not found"
>> (see msi_get_virq()), which is translated to -EINVAL by pci_irq_vector().
>>
> 
> There is a subtle difference between the description and the code of
> pci_irq_vector().
> 
> /**
>  * pci_irq_vector() - Get Linux IRQ number of a device interrupt vector
>  * @dev: the PCI device to operate on
>  * @nr:  device-relative interrupt vector index (0-based); has different
>  *       meanings, depending on interrupt mode:
>  *
>  *         * MSI-X     the index in the MSI-X vector table
>  *         * MSI       the index of the enabled MSI vectors
>  *         * INTx      must be 0
>  *
>  * Return: the Linux IRQ number, or -EINVAL if @nr is out of range
>  */
> 
> From above '0' is a valid irq number.
> 
> then in following code:
> 
> 	irq = msi_get_virq(&dev->dev, nr);
> 	return irq ? irq : -EINVAL;
> 
> '0' is obviously invalid for msi.
> 
> I didn't realize the msi part when reading the patch. It left me in
> confusion that '0' is unhandled as here we only check ">0" while in
> other places "-EINVAL" is checked.
> 
> Not big matter but it sounds slightly clearer to me to follow the
> description of pci_irq_vector() instead of its internal detail. 

I can add an explicit check for '0' and, as you confirmed, this is
invalid for MSI and thus I think it should be treated as an error.
This is perhaps another candidate for a WARN considering that
pci_irq_vector() returning a '0' for MSI indicates a kernel problem .

I now consider taking guidance from pci_irq_get_affinity(). Note that
pci_irq_get_affinity() contains:

const struct cpumask *pci_irq_get_affinity(struct pci_dev *dev, int nr)
{
	int idx, irq = pci_irq_vector(dev, nr);
	...
	if (WARN_ON_ONCE(irq <= 0))
		return NULL;
	...
}	


Would you be ok with something like below?

diff --git a/drivers/vfio/pci/vfio_pci_intrs.c b/drivers/vfio/pci/vfio_pci_intrs.c
index b549f5c97cb8..a8e96254f953 100644
--- a/drivers/vfio/pci/vfio_pci_intrs.c
+++ b/drivers/vfio/pci/vfio_pci_intrs.c
@@ -393,6 +393,8 @@ static int vfio_msi_alloc_irq(struct vfio_pci_core_device *vdev,
 	u16 cmd;
 
 	irq = pci_irq_vector(pdev, vector);
+	if (WARN_ON_ONCE(irq == 0))
+		return -EINVAL;
 	if (irq > 0 || !msix || !vdev->has_dyn_msix)
 		return irq;

I would prefer that vfio_msi_alloc_irq() returns negative errors. This enables
callers to in turn just return the error code on failure (note that dynamic
allocation can return different error codes), not needing to translate 0 into
an error.

Reinette