Re: [PATCH v3 04/41] virtio: memory access APIs

On Mon, Nov 24, 2014 at 01:58:43PM +0100, Geert Uytterhoeven wrote:

On Mon, Nov 24, 2014 at 1:15 PM, Michael S. Tsirkin [off-list ref] wrote:

quoted

On Mon, Nov 24, 2014 at 01:03:24PM +0100, Geert Uytterhoeven wrote:

quoted

On Mon, Nov 24, 2014 at 12:52 PM, Michael S. Tsirkin [off-list ref] wrote:

quoted

virtio 1.0 makes all memory structures LE, so
we need APIs to conditionally do a byteswap on BE
architectures.

To make it easier to check code statically,
add virtio specific types for multi-byte integers
in memory.

Add low level wrappers that do a byteswap conditionally, these will be
useful e.g. for vhost.  Add high level wrappers that
query device endian-ness and act accordingly.

quoted

diff --git a/include/linux/virtio_byteorder.h b/include/linux/virtio_byteorder.h
new file mode 100644
index 0000000..824ed0b
--- /dev/null
+++ b/include/linux/virtio_byteorder.h

quoted

+static inline u16 __virtio16_to_cpu(bool little_endian, __virtio16 val)
+{
+       if (little_endian)
+               return le16_to_cpu((__force __le16)val);
+       else
+               return (__force u16)val;
+}

What's wrong with just using le16-to_cpu() ...

le16-to_cpu() is simply wrong: virtio needs to be
LE or native endian, depending on whether it's running
in 0.9 or 1.0 mode.

IC, that was not clear from the description for this patch.
I thought it was dependent on BE architectures.

Nevertheless, any chance you can get rid of the "conditional"?

I don't see how - this is fundamental to any virtio device
that wants to support both 0.9 and 1.0 from the same
codebase.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds