[PATCH] virtio-iommu: Default to bypass during boot

[Jean-Philippe Brucker]

Currently the virtio-iommu device must be programmed before it allows
DMA from any PCI device. This can make the VM entirely unusable when a
virtio-iommu driver isn't present, for example in a bootloader that
loads the OS from storage.

Similarly to the other vIOMMU implementations, default to DMA bypassing
the IOMMU during boot. Add a "boot-bypass" option that lets users change
this behavior.

Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
---
 include/hw/virtio/virtio-iommu.h |  1 +
 hw/virtio/virtio-iommu.c         | 23 +++++++++++++++++++++--
 2 files changed, 22 insertions(+), 2 deletions(-)

diff --git a/include/hw/virtio/virtio-iommu.h b/include/hw/virtio/virtio-iommu.h
index 273e35c04bc..4c66989ca4e 100644
--- a/include/hw/virtio/virtio-iommu.h
+++ b/include/hw/virtio/virtio-iommu.h
@@ -58,6 +58,7 @@ struct VirtIOIOMMU {
     GTree *domains;
     QemuMutex mutex;
     GTree *endpoints;
+    bool boot_bypass;
 };
 
 #endif
diff --git a/hw/virtio/virtio-iommu.c b/hw/virtio/virtio-iommu.c
index c2883a2f6c8..cd08dc39eca 100644
--- a/hw/virtio/virtio-iommu.c
+++ b/hw/virtio/virtio-iommu.c
@@ -689,6 +689,25 @@ static void virtio_iommu_report_fault(VirtIOIOMMU *viommu, 
uint8_t reason,
 
 }
 
+static bool virtio_iommu_bypass_is_allowed(VirtIOIOMMU *s)
+{
+    VirtIODevice *vdev = &s->parent_obj;
+
+    /*
+     * Allow bypass if:
+     * - boot_bypass is enabled and the BYPASS feature hasn't yet been
+     *   acknowledged.
+     * - the BYPASS feature has been negotiated.
+     */
+    if (s->boot_bypass && !(vdev->status & VIRTIO_CONFIG_S_FEATURES_OK)) {
+        return true;
+    }
+    if (virtio_vdev_has_feature(vdev, VIRTIO_IOMMU_F_BYPASS)) {
+        return true;
+    }
+    return false;
+}
+
 static IOMMUTLBEntry virtio_iommu_translate(IOMMUMemoryRegion *mr, hwaddr addr,
                                             IOMMUAccessFlags flag,
                                             int iommu_idx)
@@ -715,8 +734,7 @@ static IOMMUTLBEntry 
virtio_iommu_translate(IOMMUMemoryRegion *mr, hwaddr addr,
         .perm = IOMMU_NONE,
     };
 
-    bypass_allowed = virtio_vdev_has_feature(&s->parent_obj,
-                                             VIRTIO_IOMMU_F_BYPASS);
+    bypass_allowed = virtio_iommu_bypass_is_allowed(s);
 
     sid = virtio_iommu_get_bdf(sdev);
 
@@ -1156,6 +1174,7 @@ static const VMStateDescription vmstate_virtio_iommu = {
 
 static Property virtio_iommu_properties[] = {
     DEFINE_PROP_LINK("primary-bus", VirtIOIOMMU, primary_bus, "PCI", PCIBus *),
+    DEFINE_PROP_BOOL("boot-bypass", VirtIOIOMMU, boot_bypass, true),
     DEFINE_PROP_END_OF_LIST(),
 };
 
-- 
2.30.0