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

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Message-ID: <20141124121503.GB14353@redhat.com>
Date:	Mon, 24 Nov 2014 14:15:03 +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 01:03:24PM +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.

> > --- a/include/uapi/linux/virtio_ring.h
> > +++ b/include/uapi/linux/virtio_ring.h
> 
> >  /* Virtio ring descriptors: 16 bytes.  These can chain together via "next". */
> >  struct vring_desc {
> >         /* Address (guest-physical). */
> > -       __u64 addr;
> > +       __virtio64 addr;
> 
> ... and __le64?
> 
> There's already lots of precedence or this, even in include/uapi/.
> 
> Gr{oetje,eeting}s,
> 
>                         Geert

__le would make people think they can use le16-to_cpu() which is wrong.

-- 
MST
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