linux-kernel - Re: [PATCH RFC don't apply] vdpa_sim: endian-ness for config space

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Date:   Thu, 16 Jul 2020 01:42:12 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH RFC don't apply] vdpa_sim: endian-ness for config space

On Wed, Jul 15, 2020 at 10:02:32PM +0800, Jason Wang wrote:
> 
> On 2020/7/15 下午9:58, Michael S. Tsirkin wrote:
> > VDPA sim stores config space as native endian, but that
> > is wrong: modern guests expect LE.
> > I coded up the following to fix it up, but it is wrong too:
> > vdpasim_create is called before guest features are known.
> > 
> > So what should we do? New ioctl to specify the interface used?
> > More ideas?
> > 
> > Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
> 
> 
> Can we do the endian conversion in set_config/get_config()?
> 
> Thanks

That is problematic at least from static checking point of view.
It would be reasonable to do it in vdpasim_set_features, except
legacy guests might not set features at all.
So my proposal is:
- set config in vdpasim_set_features
- document that this is where devices should initialize config
- vdpa core will maintain a "features set" flag, if get/set config
  is called without set features, core will call set features
  automatically with 0 value.

Thoughts?


> 
> > 
> > 
> > ---
> >   drivers/vdpa/vdpa_sim/vdpa_sim.c | 22 ++++++++++++++++++++--
> >   1 file changed, 20 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/vdpa/vdpa_sim/vdpa_sim.c b/drivers/vdpa/vdpa_sim/vdpa_sim.c
> > index a9bc5e0fb353..cc754ae0ec15 100644
> > --- a/drivers/vdpa/vdpa_sim/vdpa_sim.c
> > +++ b/drivers/vdpa/vdpa_sim/vdpa_sim.c
> > @@ -24,6 +24,7 @@
> >   #include <linux/etherdevice.h>
> >   #include <linux/vringh.h>
> >   #include <linux/vdpa.h>
> > +#include <linux/virtio_byteorder.h>
> >   #include <linux/vhost_iotlb.h>
> >   #include <uapi/linux/virtio_config.h>
> >   #include <uapi/linux/virtio_net.h>
> > @@ -72,6 +73,23 @@ struct vdpasim {
> >   	u64 features;
> >   };
> > +/* TODO: cross-endian support */
> > +static inline bool vdpasim_is_little_endian(struct vdpasim *vdpasim)
> > +{
> > +	return virtio_legacy_is_little_endian() ||
> > +		(vdpasim->features & (1ULL << VIRTIO_F_VERSION_1));
> > +}
> > +
> > +static inline u16 vdpasim16_to_cpu(struct vdpasim *vdpasim, __virtio16 val)
> > +{
> > +	return __virtio16_to_cpu(vdpasim_is_little_endian(vdpasim), val);
> > +}
> > +
> > +static inline __virtio16 cpu_to_vdpasim16(struct vdpasim *vdpasim, u16 val)
> > +{
> > +	return __cpu_to_virtio16(vdpasim_is_little_endian(vdpasim), val);
> > +}
> > +
> >   static struct vdpasim *vdpasim_dev;
> >   static struct vdpasim *vdpa_to_sim(struct vdpa_device *vdpa)
> > @@ -332,8 +350,8 @@ static struct vdpasim *vdpasim_create(void)
> >   		goto err_iommu;
> >   	config = &vdpasim->config;
> > -	config->mtu = 1500;
> > -	config->status = VIRTIO_NET_S_LINK_UP;
> > +	config->mtu = cpu_to_vdpasim16(vdpasim, 1500);
> > +	config->status = cpu_to_vdpasim16(vdpasim, VIRTIO_NET_S_LINK_UP);
> >   	eth_random_addr(config->mac);
> >   	vringh_set_iotlb(&vdpasim->vqs[0].vring, vdpasim->iommu);