Re: [Qemu-devel] [virtio-dev] Re: [PATCH v2 1/2] virtio: introduce little edian functions for virtio_cread/write# family

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Nov 28, 2016 at 04:34:56AM +0200, Michael S. Tsirkin wrote:
> On Mon, Nov 28, 2016 at 01:56:04AM +0000, Gonglei (Arei) wrote:
> > Hi Michael,
> > 
> > Thanks for your feedback firstly!
> > 
> > > -----Original Message-----
> > > From: virtio-dev@lists.oasis-open.org [mailto:virtio-dev@lists.oasis-open.org]
> > > On Behalf Of Michael S. Tsirkin
> > > Sent: Sunday, November 27, 2016 11:33 AM
> > > To: Gonglei (Arei)
> > > Subject: [virtio-dev] Re: [PATCH v2 1/2] virtio: introduce little edian functions for
> > > virtio_cread/write# family
> > > 
> > > On Tue, Nov 22, 2016 at 04:10:22PM +0800, Gonglei wrote:
> > > > Virtio modern devices are always little edian, let's introduce
> > > > the LE functions for read/write configuration space for
> > > > virtio modern devices, which avoid complaint by Sparse when
> > > > we use the virtio_creaed/virtio_cwrite in VIRTIO_1 devices.
> > > >
> > > > Signed-off-by: Gonglei <arei.gonglei@huawei.com>
> > > > ---
> > > >  include/linux/virtio_config.h | 45
> > > +++++++++++++++++++++++++++++++++++++++++++
> > > >  1 file changed, 45 insertions(+)
> > > >
> > > > diff --git a/include/linux/virtio_config.h b/include/linux/virtio_config.h
> > > > index 26c155b..de05707 100644
> > > > --- a/include/linux/virtio_config.h
> > > > +++ b/include/linux/virtio_config.h
> > > > @@ -414,4 +414,49 @@ static inline void virtio_cwrite64(struct virtio_device
> > > *vdev,
> > > >  		_r;							\
> > > >  	})
> > > >
> > > > +static inline __le16 virtio_cread16_le(struct virtio_device *vdev,
> > > > +				 unsigned int offset)
> > > > +{
> > > > +	__le16 ret;
> > > > +
> > > > +	vdev->config->get(vdev, offset, &ret, sizeof(ret));
> > > > +	return ret;
> > > > +}
> > > > +
> > > > +static inline void virtio_cwrite16_le(struct virtio_device *vdev,
> > > > +				   unsigned int offset, __le16 val)
> > > > +{
> > > > +	vdev->config->set(vdev, offset, &val, sizeof(val));
> > > > +}
> > > > +
> > > > +static inline __le32 virtio_cread32_le(struct virtio_device *vdev,
> > > > +				 unsigned int offset)
> > > > +{
> > > > +	__le32 ret;
> > > > +
> > > > +	vdev->config->get(vdev, offset, &ret, sizeof(ret));
> > > > +	return ret;
> > > > +}
> > > > +
> > > > +static inline void virtio_cwrite32_le(struct virtio_device *vdev,
> > > > +				   unsigned int offset, __le32 val)
> > > > +{
> > > > +	vdev->config->set(vdev, offset, &val, sizeof(val));
> > > > +}
> > > > +
> > > > +static inline __le64 virtio_cread64_le(struct virtio_device *vdev,
> > > > +				 unsigned int offset)
> > > > +{
> > > > +	__le64 ret;
> > > > +
> > > > +	__virtio_cread_many(vdev, offset, &ret, 1, sizeof(ret));
> > > > +	return ret;
> > > > +}
> > > > +
> > > > +static inline void virtio_cwrite64_le(struct virtio_device *vdev,
> > > > +				   unsigned int offset, __le64 val)
> > > > +{
> > > > +	vdev->config->set(vdev, offset, &val, sizeof(val));
> > > > +}
> > > > +
> > > >  #endif /* _LINUX_VIRTIO_CONFIG_H */
> > > 
> > > Could you please better explain what is the issue you are facing?
> > > virtio_cwrite/virtio_cread all accept and return native types.
> > > 
> > virtio_cwrite/virtio_cread are used to write/read configuration
> > space for virtio devices. For virtio-crypto device, I used __le32/64 directly
> > in struct virtio_crypto_config. The sparse reports warnings if I use virtio_cread()
> > for virtio-crypto device.
> 
> I suspect that's because you are doing cread into an le32 variable.
> 
> 
> 
> > Furthermore, it means the warnings exist for all VIRTIO_1 devices because
> > they are definitely LE, which it's not necessary to use virtio_to_cpu/cpu_to_virtio.
> > 
> > 
> > PS: I googled a discussion about this topic for virtio-input device, pls see:
> >  http://linux.kernel.narkive.com/3argfbWz/patch-1-1-add-virtio-input-driver
> > 
> > Regards,
> > -Gonglei
> 
> Looks like we changed the macros since - at least ATM
> virtio_console_config uses __u16 fields (which is probably a bug -
> I'll look into fixing it up) and they
> do not seem to trigger warnings.


Sorry, I recall now. We use u32 and similar types in the config
space, and endian-ness is implicit.
This works fine if you are using virtio_cread interfaces which will swap
to/from LE for you.

What doesn't work correctly is shadowing the whole config space
using ->get since that does not byte-swap. so just don't do it.

I would say just do what everyone does in this version and
we can revisit this later.

> 
> > > If you want it in LE format, swap it!
> > > 
> > > 
> > > 
> > > > --
> > > > 1.8.3.1
> > > >
> > > 
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