Date:   Tue, 7 Dec 2021 03:13:50 -0500
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     =?utf-8?B?546L6LSH?= <yun.wang@linux.alibaba.com>
Cc:     Jason Wang <jasowang@redhat.com>,
        "open list:VIRTIO CORE AND NET DRIVERS" 
        <virtualization@lists.linux-foundation.org>,
        open list <linux-kernel@vger.kernel.org>
Subject: Re: [RFC PATCH] virtio: make sure legacy pci device gain 32bit-pfn vq
Message-ID: <20211207031217-mutt-send-email-mst@kernel.org>
References: <b50fff4d-9f05-76b3-eba7-91241c351751@linux.alibaba.com>
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On Tue, Dec 07, 2021 at 03:51:45PM +0800, 王贇 wrote:
> We observed issues like:
>   virtio-pci 0000:14:00.0: platform bug: legacy virtio-mmio must
>   not be used with RAM above 0x4000GB
> 
> when we have a legacy pci device which desired 32bit-pfn vq
> but gain 64bit-pfn instead, lead into the failure of probe.
> 
> vring_use_dma_api() is playing the key role in here, to help the
> allocation process understand which kind of vq it should alloc,
> however, it failed to take care the legacy pci device, which only
> have 32bit feature flag and can never have VIRTIO_F_ACCESS_PLATFORM
> setted.
> 
> This patch introduce force_dma flag to help vring_use_dma_api()
> understanding the requirement better, to avoid the failing.
> 
> Signed-off-by: Michael Wang <yun.wang@linux.alibaba.com>

This will break configs where the device appears behind
a virtual iommu, so this won't fly.
Just make your device support 1.0, eh?

> ---
>  drivers/virtio/virtio_pci_legacy.c | 10 ++++++++++
>  drivers/virtio/virtio_ring.c       |  3 +++
>  include/linux/virtio.h             |  1 +
>  3 files changed, 14 insertions(+)
> 
> diff --git a/drivers/virtio/virtio_pci_legacy.c
> b/drivers/virtio/virtio_pci_legacy.c
> index d62e983..11f2ebf 100644
> --- a/drivers/virtio/virtio_pci_legacy.c
> +++ b/drivers/virtio/virtio_pci_legacy.c
> @@ -263,6 +263,16 @@ int virtio_pci_legacy_probe(struct virtio_pci_device
> *vp_dev)
>  	vp_dev->setup_vq = setup_vq;
>  	vp_dev->del_vq = del_vq;
> 
> +	/*
> +	 * The legacy pci device requre 32bit-pfn vq,
> +	 * or setup_vq() will failed.
> +	 *
> +	 * Thus we make sure vring_use_dma_api() will
> +	 * return true during the allocation by marking
> +	 * force_dma here.
> +	 */
> +	vp_dev->vdev.force_dma = true;
> +
>  	return 0;
> 
>  err_iomap:
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index 3035bb6..6562e01 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -245,6 +245,9 @@ static inline bool virtqueue_use_indirect(struct
> virtqueue *_vq,
> 
>  static bool vring_use_dma_api(struct virtio_device *vdev)
>  {
> +	if (vdev->force_dma)
> +		return true;
> +
>  	if (!virtio_has_dma_quirk(vdev))
>  		return true;
> 
> diff --git a/include/linux/virtio.h b/include/linux/virtio.h
> index 41edbc0..a4eb29d 100644
> --- a/include/linux/virtio.h
> +++ b/include/linux/virtio.h
> @@ -109,6 +109,7 @@ struct virtio_device {
>  	bool failed;
>  	bool config_enabled;
>  	bool config_change_pending;
> +	bool force_dma;
>  	spinlock_t config_lock;
>  	spinlock_t vqs_list_lock; /* Protects VQs list access */
>  	struct device dev;
> -- 
> 1.8.3.1