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From: Gerd Hoffmann <kraxel@redhat.com>
Date: Fri, 15 Sep 2017 09:31:20 +0200
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Subject: Re: [Qemu-devel] [PATCH v2 1/2] virtio-gpu: Handle endian conversion
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To: Farhan Ali <alifm@linux.vnet.ibm.com>, qemu-devel@nongnu.org
Cc: borntraeger@de.ibm.com, thuth@redhat.com, cohuck@redhat.com

  Hi,

> +static void
> +virtio_gpu_ctrl_hdr_bswap_cpu(struct virtio_gpu_ctrl_hdr *hdr)
> +{
> +=C2=A0=C2=A0=C2=A0=C2=A0hdr->type =3D=C2=A0=C2=A0le32_to_cpu(hdr->type=
);
> +=C2=A0=C2=A0=C2=A0=C2=A0hdr->flags =3D le32_to_cpu(hdr->flags);
> +=C2=A0=C2=A0=C2=A0=C2=A0hdr->fence_id =3D le64_to_cpu(hdr->fence_id);
> +=C2=A0=C2=A0=C2=A0=C2=A0hdr->ctx_id =3D le32_to_cpu(hdr->ctx_id);
> +=C2=A0=C2=A0=C2=A0=C2=A0hdr->padding =3D le32_to_cpu(hdr->padding);
> +}
> +
> +static void
> +virtio_gpu_ctrl_hdr_bswap_le(struct virtio_gpu_ctrl_hdr *hdr)
> +{
> +=C2=A0=C2=A0=C2=A0=C2=A0hdr->type =3D=C2=A0=C2=A0cpu_to_le32(hdr->type=
);
> +=C2=A0=C2=A0=C2=A0=C2=A0hdr->flags =3D cpu_to_le32(hdr->flags);
> +=C2=A0=C2=A0=C2=A0=C2=A0hdr->fence_id =3D cpu_to_le64(hdr->fence_id);
> +=C2=A0=C2=A0=C2=A0=C2=A0hdr->ctx_id =3D cpu_to_le32(hdr->ctx_id);
> +=C2=A0=C2=A0=C2=A0=C2=A0hdr->padding =3D cpu_to_le32(hdr->padding);
> +}

These two functions do exactly the same thing because cpu_to_le32 and
le32_to_cpu are the same operation.  The two variants only exist to
make the code more readable.  So this isn't needed twice.  I'd suggest
to just call the functions "*_bswap" and drop any _le or _cpu postfix.=20
Note that there are also variants for in-place swapping.

Have a look at the comments in include/qemu/bswap.h

> +
> +static void virtio_gpu_bswap_cpu_32(void *ptr,
> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0size=
_t size)
> +{

#ifdef HOST_WORDS_BIGENDIAN

> +=C2=A0=C2=A0=C2=A0=C2=A0size_t i;
> +=C2=A0=C2=A0=C2=A0=C2=A0struct virtio_gpu_ctrl_hdr *hdr =3D (struct vi=
rtio_gpu_ctrl_hdr *)
> ptr;
> +
> +=C2=A0=C2=A0=C2=A0=C2=A0virtio_gpu_ctrl_hdr_bswap_cpu(hdr);
> +
> +=C2=A0=C2=A0=C2=A0=C2=A0i =3D sizeof(struct virtio_gpu_ctrl_hdr);
> +=C2=A0=C2=A0=C2=A0=C2=A0while (i < size) {
> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0*(uint32_t *)(ptr + i)=
 =3D le32_to_cpu(*(uint32_t *)(ptr +
> i));
> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0i =3D i + sizeof(uint3=
2_t);
> +=C2=A0=C2=A0=C2=A0=C2=A0}

#endif

I suspect the compiler isn't clever enough to figure the whole function
is a nop on little endian hosts.

cheers,
  Gerd