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

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Date:	Mon, 24 Nov 2014 20:49:09 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	David Miller <davem@...emloft.net>, cornelia.huck@...ibm.com,
	rusty@....ibm.com, Nicholas Bellinger <nab@...ux-iscsi.org>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Pranavkumar Sawargaonkar <pranavkumar@...aro.org>,
	Anup Patel <anup.patel@...aro.org>,
	Sakari Ailus <sakari.ailus@...ux.intel.com>,
	Bjarke Istrup Pedersen <gurligebis@...too.org>,
	stephen hemminger <stephen@...workplumber.org>,
	virtualization@...ts.linux-foundation.org,
	linux-api@...r.kernel.org
Subject: 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 <mst@...hat.com> wrote:
> > On Mon, Nov 24, 2014 at 01:03:24PM +0100, Geert Uytterhoeven wrote:
> >> On Mon, Nov 24, 2014 at 12:52 PM, Michael S. Tsirkin <mst@...hat.com> wrote:
> >> > 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.
> >>
> >> > 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
> >>
> >> > +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@...ux-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
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