Re: [PATCH 7/9] virtio-pci: harden INTX interrupts

From: "Michael S. Tsirkin" <mst@redhat.com>
To: Jason Wang <jasowang@redhat.com>
Cc: konrad.wilk@oracle.com, f.hetzelt@tu-berlin.de,
	linux-kernel@vger.kernel.org, david.kaplan@amd.com,
	virtualization@lists.linux-foundation.org
Subject: Re: [PATCH 7/9] virtio-pci: harden INTX interrupts
Date: Mon, 13 Sep 2021 02:33:25 -0400	[thread overview]
Message-ID: <20210913022824-mutt-send-email-mst@kernel.org> (raw)
In-Reply-To: <20210913055353.35219-8-jasowang@redhat.com>

On Mon, Sep 13, 2021 at 01:53:51PM +0800, Jason Wang wrote:
> This patch tries to make sure the virtio interrupt handler for INTX
> won't be called after a reset and before virtio_device_ready(). We
> can't use IRQF_NO_AUTOEN since we're using shared interrupt
> (IRQF_SHARED). So this patch tracks the INTX enabling status in a new
> intx_soft_enabled variable and toggle it during in
> vp_disable/enable_vectors(). The INTX interrupt handler will check
> intx_soft_enabled before processing the actual interrupt.
> 
> Signed-off-by: Jason Wang <jasowang@redhat.com>

Not all that excited about all the memory barriers for something
that should be an extremely rare event (for most kernels -
literally once per boot). Can't we do better?

> ---
>  drivers/virtio/virtio_pci_common.c | 18 ++++++++++++++++--
>  drivers/virtio/virtio_pci_common.h |  1 +
>  2 files changed, 17 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/virtio/virtio_pci_common.c b/drivers/virtio/virtio_pci_common.c
> index 0b9523e6dd39..835197151dc1 100644
> --- a/drivers/virtio/virtio_pci_common.c
> +++ b/drivers/virtio/virtio_pci_common.c
> @@ -30,8 +30,12 @@ void vp_disable_vectors(struct virtio_device *vdev)
>  	struct virtio_pci_device *vp_dev = to_vp_device(vdev);
>  	int i;
>  
> -	if (vp_dev->intx_enabled)
> +	if (vp_dev->intx_enabled) {
> +		vp_dev->intx_soft_enabled = false;
> +		/* ensure the vp_interrupt see this intx_soft_enabled value */
> +		smp_wmb();
>  		synchronize_irq(vp_dev->pci_dev->irq);
> +	}
>  
>  	for (i = 0; i < vp_dev->msix_vectors; ++i)
>  		disable_irq(pci_irq_vector(vp_dev->pci_dev, i));
> @@ -43,8 +47,12 @@ void vp_enable_vectors(struct virtio_device *vdev)
>  	struct virtio_pci_device *vp_dev = to_vp_device(vdev);
>  	int i;
>  
> -	if (vp_dev->intx_enabled)
> +	if (vp_dev->intx_enabled) {
> +		vp_dev->intx_soft_enabled = true;
> +		/* ensure the vp_interrupt see this intx_soft_enabled value */
> +		smp_wmb();
>  		return;
> +	}
>  
>  	for (i = 0; i < vp_dev->msix_vectors; ++i)
>  		enable_irq(pci_irq_vector(vp_dev->pci_dev, i));
> @@ -97,6 +105,12 @@ static irqreturn_t vp_interrupt(int irq, void *opaque)
>  	struct virtio_pci_device *vp_dev = opaque;
>  	u8 isr;
>  
> +	if (!vp_dev->intx_soft_enabled)
> +		return IRQ_NONE;
> +
> +	/* read intx_soft_enabled before read others */
> +	smp_rmb();
> +
>  	/* reading the ISR has the effect of also clearing it so it's very
>  	 * important to save off the value. */
>  	isr = ioread8(vp_dev->isr);
> diff --git a/drivers/virtio/virtio_pci_common.h b/drivers/virtio/virtio_pci_common.h
> index a235ce9ff6a5..3c06e0f92ee4 100644
> --- a/drivers/virtio/virtio_pci_common.h
> +++ b/drivers/virtio/virtio_pci_common.h
> @@ -64,6 +64,7 @@ struct virtio_pci_device {
>  	/* MSI-X support */
>  	int msix_enabled;
>  	int intx_enabled;
> +	bool intx_soft_enabled;
>  	cpumask_var_t *msix_affinity_masks;
>  	/* Name strings for interrupts. This size should be enough,
>  	 * and I'm too lazy to allocate each name separately. */
> -- 
> 2.25.1

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