[PATCH] virtio: Prevent double swap due to target pre 1.0 VirtIO

[Andre Silva <afscoelho@gmail.com>]

Remove the bswap function calls after reading and before writing
memory bytes in virtio_pci_config_read and virtio_pci_config_write
because they are reverting back an already swapped bytes.

Consider the table below in the context of virtio_pci_config_read
function.

Host   Target  virtio-config-read[wl]
               swap?                   virtio-is-big-endian?   extra bswap?   Should be   Final result   Final result ok?
----- ------- ------------------------ ----------------------- -------------- ----------- -------------- ------------------
LE     BE      s(x)                    true                    s(s(x))        s(x)        x              No
LE     LE      x                       false                   -              x           x              Yes
BE     LE      s(x)                    false                   -              s(x)        s(x)           Yes
BE     BE      x                       true                    s(x)           x           s(x)           No

In table above, when target is big endian and VirtIO is pre 1.0,
function virtio_is_big_endian would return true and the extra
swap would be executed, reverting the previous swap made by
virtio_config_read[wl].

The 's(x)' means that a swap function was applied at
address x. 'LE' is little endian and 'BE' is big endian. The
'Final result' column is the returned value from
virtio_pci_config_read, considering a target Virtio pre 1.0.
'x' means that target's value was not swapped in Qemu, 's(x)' means
that Qemu will use a swapped value.

If we remove the extra swap made in virtio_pci_config_read we will
have the correct result in any host/target combination, both for
VirtIO pre 1.0 or later versions.

The same reasoning applies to virtio_pci_config_write.
---
 hw/virtio/virtio-pci.c | 12 ------------
 1 file changed, 12 deletions(-)

diff --git a/hw/virtio/virtio-pci.c b/hw/virtio/virtio-pci.c
index c6b47a9c73..4ba9e847f3 100644
--- a/hw/virtio/virtio-pci.c
+++ b/hw/virtio/virtio-pci.c
@@ -431,15 +431,9 @@ 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(vdev)) {
-            val = bswap16(val);
-        }
         break;
     case 4:
         val = virtio_config_readl(vdev, addr);
-        if (virtio_is_big_endian(vdev)) {
-            val = bswap32(val);
-        }
         break;
     }
     return val;
@@ -465,15 +459,9 @@ static void virtio_pci_config_write(void *opaque, hwaddr addr,
         virtio_config_writeb(vdev, addr, val);
         break;
     case 2:
-        if (virtio_is_big_endian(vdev)) {
-            val = bswap16(val);
-        }
         virtio_config_writew(vdev, addr, val);
         break;
     case 4:
-        if (virtio_is_big_endian(vdev)) {
-            val = bswap32(val);
-        }
         virtio_config_writel(vdev, addr, val);
         break;
     }
-- 
2.24.1