[PATCH v5 09/10] vring: Use the DMA API on Xen

[Andy Lutomirski]

Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
---
 drivers/virtio/virtio_ring.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index c169c6444637..305c05cc249a 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -47,6 +47,18 @@
 
 static bool vring_use_dma_api(void)
 {
+#if defined(CONFIG_X86) && defined(CONFIG_XEN)
+	/*
+	 * In theory, it's possible to have a buggy QEMU-supposed
+	 * emulated Q35 IOMMU and Xen enabled at the same time.  On
+	 * such a configuration, virtio has never worked and will
+	 * not work without an even larger kludge.  Instead, enable
+	 * the DMA API if we're a Xen guest, which at least allows
+	 * all of the sensible Xen configurations to work correctly.
+	 */
+	return static_cpu_has(X86_FEATURE_XENPV);
+#endif
+
 	return false;
 }
 
-- 
2.5.0