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Date:   Wed, 5 Aug 2020 08:06:27 -0400
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     Jason Wang <jasowang@redhat.com>
Cc:     linux-kernel@vger.kernel.org,
        virtualization@lists.linux-foundation.org
Subject: Re: [PATCH v2 22/24] vdpa_sim: fix endian-ness of config space
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On Wed, Aug 05, 2020 at 02:21:07PM +0800, Jason Wang wrote:
> 
> On 2020/8/4 上午5:00, Michael S. Tsirkin wrote:
> > VDPA sim accesses config space as native endian - this is
> > wrong since it's a modern device and actually uses LE.
> > 
> > It only supports modern guests so we could punt and
> > just force LE, but let's use the full virtio APIs since people
> > tend to copy/paste code, and this is not data path anyway.
> > 
> > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> > ---
> >   drivers/vdpa/vdpa_sim/vdpa_sim.c | 31 ++++++++++++++++++++++++++-----
> >   1 file changed, 26 insertions(+), 5 deletions(-)
> > 
> > diff --git a/drivers/vdpa/vdpa_sim/vdpa_sim.c b/drivers/vdpa/vdpa_sim/vdpa_sim.c
> > index a9bc5e0fb353..fa05e065ff69 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)
> > @@ -306,7 +324,6 @@ static const struct vdpa_config_ops vdpasim_net_config_ops;
> >   static struct vdpasim *vdpasim_create(void)
> >   {
> > -	struct virtio_net_config *config;
> >   	struct vdpasim *vdpasim;
> >   	struct device *dev;
> >   	int ret = -ENOMEM;
> > @@ -331,10 +348,7 @@ static struct vdpasim *vdpasim_create(void)
> >   	if (!vdpasim->buffer)
> >   		goto err_iommu;
> > -	config = &vdpasim->config;
> > -	config->mtu = 1500;
> > -	config->status = VIRTIO_NET_S_LINK_UP;
> > -	eth_random_addr(config->mac);
> > +	eth_random_addr(vdpasim->config.mac);
> >   	vringh_set_iotlb(&vdpasim->vqs[0].vring, vdpasim->iommu);
> >   	vringh_set_iotlb(&vdpasim->vqs[1].vring, vdpasim->iommu);
> > @@ -448,6 +462,7 @@ static u64 vdpasim_get_features(struct vdpa_device *vdpa)
> >   static int vdpasim_set_features(struct vdpa_device *vdpa, u64 features)
> >   {
> >   	struct vdpasim *vdpasim = vdpa_to_sim(vdpa);
> > +	struct virtio_net_config *config = &vdpasim->config;
> >   	/* DMA mapping must be done by driver */
> >   	if (!(features & (1ULL << VIRTIO_F_ACCESS_PLATFORM)))
> > @@ -455,6 +470,12 @@ static int vdpasim_set_features(struct vdpa_device *vdpa, u64 features)
> >   	vdpasim->features = features & vdpasim_features;
> > +	/* We only know whether guest is using the legacy interface here, so
> > +	 * that's the earliest we can set config fields.
> > +	 */
> 
> 
> We check whether or not ACCESS_PLATFORM is set before which is probably a
> hint that only modern device is supported. So I wonder just force LE and
> fail if VERSION_1 is not set is better?
> 
> Thanks

So how about I add a comment along the lines of

/*
 * vdpasim ATM requires VIRTIO_F_ACCESS_PLATFORM, so we don't need to
 * support legacy guests. Keep transitional device code around for
 * the benefit of people who might copy-and-paste this into transitional
 * device code.
 */


> 
> > +
> > +	config->mtu = cpu_to_vdpasim16(vdpasim, 1500);
> > +	config->status = cpu_to_vdpasim16(vdpasim, VIRTIO_NET_S_LINK_UP);
> >   	return 0;
> >   }