[RFC PATCH] virtio: make sure legacy pci device gain 32bit-pfn vq

[王贇 <yun.wang@linux.alibaba.com>]

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>
---
  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