Re: [PATCH] tests/qtest/libqos: Avoid double swapping when using modern virtio

["Philippe Mathieu-Daudé" <philmd@linaro.org>]

On 30/4/25 09:33, Thomas Huth wrote:
> From: Thomas Huth <thuth@redhat.com>
> 
> The logic in the qvirtio_read/write function is rather a headache,
> involving byte-swapping when the target is big endian, just to
> maybe involve another byte-swapping  in the qtest_read/write
> function immediately afterwards (on the QEMU side). Let's do it in
> a more obvious way here: For virtio 1.0, we know that the values have
> to be little endian, so let's read/write the bytes in that well known
> order here.

Thanks for looking at this!

> 
> Signed-off-by: Thomas Huth <thuth@redhat.com>
> ---
>   This also decreases our usage of qtest_big_endian() which might (or
>   might not) get helpful for the universal binary one day...
> 
>   tests/qtest/libqos/virtio.c | 61 ++++++++++++++++++++++++++-----------
>   1 file changed, 44 insertions(+), 17 deletions(-)
> 
> diff --git a/tests/qtest/libqos/virtio.c b/tests/qtest/libqos/virtio.c
> index 2e7979652fd..078adf3c8dc 100644
> --- a/tests/qtest/libqos/virtio.c
> +++ b/tests/qtest/libqos/virtio.c
> @@ -25,49 +25,76 @@
>    */
>   static uint16_t qvirtio_readw(QVirtioDevice *d, QTestState *qts, uint64_t addr)
>   {
> -    uint16_t val = qtest_readw(qts, addr);
> +    uint8_t buf[2];
>   
> -    if (d->features & (1ull << VIRTIO_F_VERSION_1) && qtest_big_endian(qts)) {
> -        val = bswap16(val);
> +    if (d->features & (1ull << VIRTIO_F_VERSION_1)) {
> +        qtest_memread(qts, addr, buf, sizeof(buf));
> +        return (buf[1] << 8) | buf[0];
> +    } else {
> +        return qtest_readw(qts, addr);
>       }
> -    return val;
>   }

What about using cpu_to_le() API?

-- >8 --
  static uint16_t qvirtio_readw(QVirtioDevice *d, QTestState *qts, 
uint64_t addr)
  {
-    uint16_t val = qtest_readw(qts, addr);
+    uint16_t val;

-    if (d->features & (1ull << VIRTIO_F_VERSION_1) && 
qtest_big_endian(qts)) {
-        val = bswap16(val);
+    if (d->features & (1ull << VIRTIO_F_VERSION_1)) {
+        qtest_memread(qts, addr, &val, sizeof(val));
+        cpu_to_le16s(&val);
+    } else {
+        val = qtest_readw(qts, addr);
      }
+
      return val;
  }
---