[Qemu-devel] [PATCH v2 RFC] virtio-pci: fix LE host/BE guest capacity for blk

["Michael S. Tsirkin" <mst@redhat.com>]

When a BE guest reads capacity from an LE host virtio-blk device or vice
versa, it will get the dwords of the qword field swapped.
As virtio-blk is the only one with such a quirk,
and as non-pci transports don't do byte-swaps at all,
solve this with a bit of device-specific hackery in
virtio-pci.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---

Changes from v1:
fixed some obvious bugs.

I don't seem to be able to boot any big-endian
guests ATM, so this is only compile-tested - sending
this out for early feedback/flames.
Testing reports also wellcome!


diff --git a/hw/virtio/virtio-pci.c b/hw/virtio/virtio-pci.c
index 9668b2b..e7971cc 100644
--- a/hw/virtio/virtio-pci.c
+++ b/hw/virtio/virtio-pci.c
@@ -98,6 +98,12 @@
 /* HACK for virtio to determine if it's running a big endian guest */
 bool virtio_is_big_endian(void);
 
+#ifdef HOST_WORDS_BIGENDIAN
+#define VIRTIO_HOST_IS_BIG_ENDIAN true
+#else
+#define VIRTIO_HOST_IS_BIG_ENDIAN false
+#endif
+
 static void virtio_pci_bus_new(VirtioBusState *bus, VirtIOPCIProxy *dev);
 
 /* virtio device */
@@ -411,6 +417,15 @@ static uint64_t virtio_pci_config_read(void *opaque, hwaddr addr,
         }
         break;
     case 4:
+        /* Most devices don't have 64 bit config fields.
+         * Block is an exception: first 8 bytes include
+         * a 64 bit capacity field.
+         */
+        if (virtio_is_big_endian() != VIRTIO_HOST_IS_BIG_ENDIAN &&
+            proxy->vdev->device_id == VIRTIO_ID_BLOCK && addr < 8) {
+            /* Swap first two words */
+            addr ^= 0x4;
+        }
         val = virtio_config_readl(proxy->vdev, addr);
         if (virtio_is_big_endian()) {
             val = bswap32(val);