vdpa/vp_vdpa: Wait for device reset to complete

vp_vdpa_reset() writes 0 to device_status and returns success right away. The virtio specification requires the driver to wait until a read of device_status returns 0 before reinitializing the device, and virtio_pci_modern's vp_reset() does exactly that: the read-back flushes the posted status write and any in-flight device writes, including MSI-X messages.

Without it, vhost-vdpa (or virtio-vdpa) can go on to tear down the IOTLB/IOMMU mappings or reprogram the queues while the device has not yet finished its reset, and may still be DMAing to the old virtqueue addresses. The MSI-X vectors are also freed while an interrupt from the old configuration may still be in flight.

Poll device_status after writing 0, in the same way as vp_reset(), before freeing the IRQs and returning.

---
Compile-tested only (x86_64 defconfig + VP_VDPA, W=1).

 drivers/vdpa/virtio_pci/vp_vdpa.c | 4 ++++
 1 file changed, 4 insertions(+)

index f2eb654b1665..4285cb968479 100644
@@ -8,6 +8,7 @@
  * Based on virtio_pci_modern.c.
  */
 
+#include <linux/delay.h>
 #include <linux/interrupt.h>
 #include <linux/module.h>
 #include <linux/pci.h>

@@ -236,6 +237,9 @@ static int vp_vdpa_reset(struct vdpa_device *vdpa)
 	u8 s = vp_vdpa_get_status(vdpa);
 
 	vp_modern_set_status(mdev, 0);
+	/* Wait for the reset to complete */
+	while (vp_modern_get_status(mdev))
+		fsleep(1000);
 
 	if (s & VIRTIO_CONFIG_S_DRIVER_OK)
 		vp_vdpa_free_irq(vp_vdpa);

base-commit: aa98230e410f0ed212b6788c46b1e4d49e0ff7ca

2.43.0