[Qemu-devel] [PATCH RFC 8/8] virtio: add endian-ambivalent support to VirtIODevice

[Greg Kurz]

Some CPU families can dynamically change their endianness. This means we
can have little endian ppc or big endian arm guests for example. This has
an impact on legacy virtio data structures since they are target endian.
We hence introduce a new property to track the endianness of each virtio
device. It is reasonnably assumed that endianness won't change while the
device is in use : we hence capture the device endianness when it gets
reset.

Of course this property must be part of the migration stream as most of
the virtio code will depend on it. It is hence migrated in a subsection
to preserve compatibility with older migration streams. The tricky part
is that the endianness gets known quite late and we must ensure no access
is made to virtio data before that. It is for example invalid to call
vring_avail_idx() as does the actual migration code: the VQ indexes sanity
checks had to be moved from virtio_load() to virtio_load_subsections()
because of that.

We enforce some paranoia by making the endianness a 3-value enum so
that we can temporarily poison it while loading state.

Signed-off-by: Greg Kurz <address@hidden>
---
 exec.c                     |    8 +---
 hw/virtio/virtio-pci.c     |   11 ++----
 hw/virtio/virtio.c         |   80 +++++++++++++++++++++++++++++++++++++-------
 include/exec/cpu-common.h  |    1 +
 include/hw/virtio/virtio.h |   13 +++++++
 5 files changed, 87 insertions(+), 26 deletions(-)

diff --git a/exec.c b/exec.c
index 91513c6..4e83588 100644
--- a/exec.c
+++ b/exec.c
@@ -2740,13 +2740,7 @@ int cpu_memory_rw_debug(CPUState *cpu, target_ulong addr,
 #endif
 
 #if !defined(CONFIG_USER_ONLY)
-
-/*
- * A helper function for the _utterly broken_ virtio device model to find out 
if
- * it's running on a big endian machine. Don't do this at home kids!
- */
-bool virtio_is_big_endian(void);
-bool virtio_is_big_endian(void)
+bool target_words_bigendian(void)
 {
 #if defined(TARGET_WORDS_BIGENDIAN)
     return true;
diff --git a/hw/virtio/virtio-pci.c b/hw/virtio/virtio-pci.c
index ce97514..2ffceb8 100644
--- a/hw/virtio/virtio-pci.c
+++ b/hw/virtio/virtio-pci.c
@@ -89,9 +89,6 @@
 /* Flags track per-device state like workarounds for quirks in older guests. */
 #define VIRTIO_PCI_FLAG_BUS_MASTER_BUG  (1 << 0)
 
-/* HACK for virtio to determine if it's running a big endian guest */
-bool virtio_is_big_endian(void);
-
 static void virtio_pci_bus_new(VirtioBusState *bus, size_t bus_size,
                                VirtIOPCIProxy *dev);
 
@@ -409,13 +406,13 @@ static uint64_t virtio_pci_config_read(void *opaque, 
hwaddr addr,
         break;
     case 2:
         val = virtio_config_readw(vdev, addr);
-        if (virtio_is_big_endian()) {
+        if (virtio_is_big_endian(vdev)) {
             val = bswap16(val);
         }
         break;
     case 4:
         val = virtio_config_readl(vdev, addr);
-        if (virtio_is_big_endian()) {
+        if (virtio_is_big_endian(vdev)) {
             val = bswap32(val);
         }
         break;
@@ -443,13 +440,13 @@ static void virtio_pci_config_write(void *opaque, hwaddr 
addr,
         virtio_config_writeb(vdev, addr, val);
         break;
     case 2:
-        if (virtio_is_big_endian()) {
+        if (virtio_is_big_endian(vdev)) {
             val = bswap16(val);
         }
         virtio_config_writew(vdev, addr, val);
         break;
     case 4:
-        if (virtio_is_big_endian()) {
+        if (virtio_is_big_endian(vdev)) {
             val = bswap32(val);
         }
         virtio_config_writel(vdev, addr, val);
diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
index 7fbad29..6578854 100644
--- a/hw/virtio/virtio.c
+++ b/hw/virtio/virtio.c
@@ -539,6 +539,15 @@ void virtio_set_status(VirtIODevice *vdev, uint8_t val)
     vdev->status = val;
 }
 
+static void virtio_set_endian_target_default(VirtIODevice *vdev)
+{
+    if (target_words_bigendian()) {
+        vdev->device_endian = VIRTIO_DEVICE_ENDIAN_BIG;
+    } else {
+        vdev->device_endian = VIRTIO_DEVICE_ENDIAN_LITTLE;
+    }
+}
+
 void virtio_reset(void *opaque)
 {
     VirtIODevice *vdev = opaque;
@@ -546,6 +555,7 @@ void virtio_reset(void *opaque)
     int i;
 
     virtio_set_status(vdev, 0);
+    virtio_set_endian_target_default(vdev);
 
     if (k->reset) {
         k->reset(vdev);
@@ -839,10 +849,39 @@ typedef struct VirtIOSubsection {
     int version_id;
     void (*save)(VirtIODevice *vdev, QEMUFile *f);
     int (*load)(VirtIODevice *vdev, QEMUFile *f);
-    int (*needed)(VirtIODevice *vdev);
+    bool (*needed)(VirtIODevice *vdev);
 } VirtIOSubsection;
 
+static void virtio_save_device_endian(VirtIODevice *vdev, QEMUFile *f)
+{
+    qemu_put_byte(f, vdev->device_endian);
+}
+
+static int virtio_load_device_endian(VirtIODevice *vdev, QEMUFile *f)
+{
+    vdev->device_endian = qemu_get_byte(f);
+    return 0;
+}
+
+static bool virtio_device_endian_needed(VirtIODevice *vdev)
+{
+    /* No migration is supposed to occur while we are loading state.
+     */
+    assert(vdev->device_endian != VIRTIO_DEVICE_ENDIAN_UNKNOWN);
+    if (target_words_bigendian()) {
+        return vdev->device_endian == VIRTIO_DEVICE_ENDIAN_LITTLE;
+    } else {
+        return vdev->device_endian == VIRTIO_DEVICE_ENDIAN_BIG;
+    }
+}
+
 static const VirtIOSubsection virtio_subsection[] = {
+    { .name = "virtio/device_endian",
+      .version_id = 1,
+      .save = virtio_save_device_endian,
+      .load = virtio_load_device_endian,
+      .needed = virtio_device_endian_needed,
+    },
     { .name = NULL }
 };
 
@@ -868,6 +907,8 @@ static void virtio_save_subsections(VirtIODevice *vdev, 
QEMUFile *f)
 
 static int virtio_load_subsections(VirtIODevice *vdev, QEMUFile *f)
 {
+    int i;
+
     while (qemu_peek_byte(f, 0) == QEMU_VM_SUBSECTION) {
         char idstr[256];
         uint8_t len, size;
@@ -904,6 +945,26 @@ static int virtio_load_subsections(VirtIODevice *vdev, 
QEMUFile *f)
         }
     }
 
+    for (i = 0; i < VIRTIO_PCI_QUEUE_MAX; i++) {
+        if (vdev->vq[i].vring.num == 0) {
+            break;
+        }
+
+        if (vdev->vq[i].pa) {
+            uint16_t nheads;
+            nheads = vring_avail_idx(&vdev->vq[i]) - 
vdev->vq[i].last_avail_idx;
+            /* Check it isn't doing strange things with descriptor numbers. */
+            if (nheads > vdev->vq[i].vring.num) {
+                error_report("VQ %d size 0x%x Guest index 0x%x "
+                             "inconsistent with Host index 0x%x: delta 0x%x",
+                             i, vdev->vq[i].vring.num,
+                             vring_avail_idx(&vdev->vq[i]),
+                             vdev->vq[i].last_avail_idx, nheads);
+                return -1;
+            }
+        }
+    }
+
     return 0;
 }
 
@@ -979,6 +1040,11 @@ int virtio_load(VirtIODevice *vdev, QEMUFile *f, int 
version_id)
     VirtioBusClass *k = VIRTIO_BUS_GET_CLASS(qbus);
     VirtioDeviceClass *vdc = VIRTIO_DEVICE_GET_CLASS(vdev);
 
+    /* We poison the endianness to ensure it does not get used before
+     * subsections have been loaded.
+     */
+    vdev->device_endian = VIRTIO_DEVICE_ENDIAN_UNKNOWN;
+
     if (k->load_config) {
         ret = k->load_config(qbus->parent, f);
         if (ret)
@@ -1012,18 +1078,7 @@ int virtio_load(VirtIODevice *vdev, QEMUFile *f, int 
version_id)
         vdev->vq[i].notification = true;
 
         if (vdev->vq[i].pa) {
-            uint16_t nheads;
             virtqueue_init(&vdev->vq[i]);
-            nheads = vring_avail_idx(&vdev->vq[i]) - 
vdev->vq[i].last_avail_idx;
-            /* Check it isn't doing very strange things with descriptor 
numbers. */
-            if (nheads > vdev->vq[i].vring.num) {
-                error_report("VQ %d size 0x%x Guest index 0x%x "
-                             "inconsistent with Host index 0x%x: delta 0x%x",
-                             i, vdev->vq[i].vring.num,
-                             vring_avail_idx(&vdev->vq[i]),
-                             vdev->vq[i].last_avail_idx, nheads);
-                return -1;
-            }
         } else if (vdev->vq[i].last_avail_idx) {
             error_report("VQ %d address 0x0 "
                          "inconsistent with Host index 0x%x",
@@ -1100,6 +1155,7 @@ void virtio_init(VirtIODevice *vdev, const char *name,
     }
     vdev->vmstate = qemu_add_vm_change_state_handler(virtio_vmstate_change,
                                                      vdev);
+    virtio_set_endian_target_default(vdev);
 }
 
 hwaddr virtio_queue_get_desc_addr(VirtIODevice *vdev, int n)
diff --git a/include/exec/cpu-common.h b/include/exec/cpu-common.h
index a21b65a..eb798c1 100644
--- a/include/exec/cpu-common.h
+++ b/include/exec/cpu-common.h
@@ -122,4 +122,5 @@ void qemu_ram_foreach_block(RAMBlockIterFunc func, void 
*opaque);
 
 #endif
 
+bool target_words_bigendian(void);
 #endif /* !CPU_COMMON_H */
diff --git a/include/hw/virtio/virtio.h b/include/hw/virtio/virtio.h
index 3505ce5..1c4a736 100644
--- a/include/hw/virtio/virtio.h
+++ b/include/hw/virtio/virtio.h
@@ -104,6 +104,12 @@ typedef struct VirtQueueElement
 #define VIRTIO_DEVICE(obj) \
         OBJECT_CHECK(VirtIODevice, (obj), TYPE_VIRTIO_DEVICE)
 
+enum virtio_device_endian {
+    VIRTIO_DEVICE_ENDIAN_UNKNOWN,
+    VIRTIO_DEVICE_ENDIAN_LITTLE,
+    VIRTIO_DEVICE_ENDIAN_BIG,
+};
+
 struct VirtIODevice
 {
     DeviceState parent_obj;
@@ -121,6 +127,7 @@ struct VirtIODevice
     bool vm_running;
     VMChangeStateEntry *vmstate;
     char *bus_name;
+    enum virtio_device_endian device_endian;
 };
 
 typedef struct VirtioDeviceClass {
@@ -255,4 +262,10 @@ void virtio_queue_set_host_notifier_fd_handler(VirtQueue 
*vq, bool assign,
                                                bool set_handler);
 void virtio_queue_notify_vq(VirtQueue *vq);
 void virtio_irq(VirtQueue *vq);
+
+static inline bool virtio_is_big_endian(VirtIODevice *vdev)
+{
+    assert(vdev->device_endian != VIRTIO_DEVICE_ENDIAN_UNKNOWN);
+    return vdev->device_endian == VIRTIO_DEVICE_ENDIAN_BIG;
+}
 #endif