Re: [PATCH v2 1/7] vfio/pci: Disable auto-enable of exclusive INTx IRQ

From: Eric Auger <eric.auger@redhat.com>
To: Alex Williamson <alex.williamson@redhat.com>
Cc: kvm@vger.kernel.org, clg@redhat.com, reinette.chatre@intel.com,
	linux-kernel@vger.kernel.org, kevin.tian@intel.com,
	stable@vger.kernel.org
Subject: Re: [PATCH v2 1/7] vfio/pci: Disable auto-enable of exclusive INTx IRQ
Date: Mon, 11 Mar 2024 08:36:07 +0100	[thread overview]
Message-ID: <d57f0df4-b155-4805-9121-53a9a1c23cee@redhat.com> (raw)
In-Reply-To: <20240308230557.805580-2-alex.williamson@redhat.com>

Hi Alex,

On 3/9/24 00:05, Alex Williamson wrote:
> Currently for devices requiring masking at the irqchip for INTx, ie.
> devices without DisINTx support, the IRQ is enabled in request_irq()
> and subsequently disabled as necessary to align with the masked status
> flag.  This presents a window where the interrupt could fire between
> these events, resulting in the IRQ incrementing the disable depth twice.
> This would be unrecoverable for a user since the masked flag prevents
> nested enables through vfio.
>
> Instead, invert the logic using IRQF_NO_AUTOEN such that exclusive INTx
> is never auto-enabled, then unmask as required.
> Cc: stable@vger.kernel.org
> Fixes: 89e1f7d4c66d ("vfio: Add PCI device driver")
> Reviewed-by: Kevin Tian <kevin.tian@intel.com>
> Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
> ---
>  drivers/vfio/pci/vfio_pci_intrs.c | 17 ++++++++++-------
>  1 file changed, 10 insertions(+), 7 deletions(-)
>
> diff --git a/drivers/vfio/pci/vfio_pci_intrs.c b/drivers/vfio/pci/vfio_pci_intrs.c
> index 237beac83809..136101179fcb 100644
> --- a/drivers/vfio/pci/vfio_pci_intrs.c
> +++ b/drivers/vfio/pci/vfio_pci_intrs.c
> @@ -296,8 +296,15 @@ static int vfio_intx_set_signal(struct vfio_pci_core_device *vdev, int fd)
>  
>  	ctx->trigger = trigger;
>  
> +	/*
> +	 * Devices without DisINTx support require an exclusive interrupt,
> +	 * IRQ masking is performed at the IRQ chip.  The masked status is
> +	 * protected by vdev->irqlock. Setup the IRQ without auto-enable and
> +	 * unmask as necessary below under lock.  DisINTx is unmodified by
> +	 * the IRQ configuration and may therefore use auto-enable.
If I remember correctly the main reason why the

vdev->pci_2_3 path is left unchanged is due to the fact the irq may not be exclusive
and setting IRQF_NO_AUTOEN could be wrong in that case. May be worth to
precise in the commit msg or here? Besides Reviewed-by: Eric Auger
<eric.auger@redhat.com> Eric   

> +	 */
>  	if (!vdev->pci_2_3)
> -		irqflags = 0;
> +		irqflags = IRQF_NO_AUTOEN;
>  
>  	ret = request_irq(pdev->irq, vfio_intx_handler,
>  			  irqflags, ctx->name, vdev);
> @@ -308,13 +315,9 @@ static int vfio_intx_set_signal(struct vfio_pci_core_device *vdev, int fd)
>  		return ret;
>  	}
>  
> -	/*
> -	 * INTx disable will stick across the new irq setup,
> -	 * disable_irq won't.
> -	 */
>  	spin_lock_irqsave(&vdev->irqlock, flags);
> -	if (!vdev->pci_2_3 && ctx->masked)
> -		disable_irq_nosync(pdev->irq);
> +	if (!vdev->pci_2_3 && !ctx->masked)
> +		enable_irq(pdev->irq);
>  	spin_unlock_irqrestore(&vdev->irqlock, flags);
>  
>  	return 0;