Re: [PATCH v2 7/7] vhost: feature to set the vring endianness

From: Greg Kurz
Date: Thu Apr 02 2015 - 12:45:53 EST

On Thu, 2 Apr 2015 16:20:46 +0200
"Michael S. Tsirkin" <mst@xxxxxxxxxx> wrote:

> On Thu, Apr 02, 2015 at 03:17:13PM +0200, Greg Kurz wrote:
> > This patch brings cross-endian support to vhost when used to implement
> > legacy virtio devices. Since it is a relatively rare situation, the feature
> > availability is controlled by a kernel config option (not set by default).
> >
> > If cross-endian support is compiled in, vhost abvertises a new feature
> > to be negotiated with userspace. If userspace acknowledges the feature,
> > it can inform vhost about the endianness to use with a new ioctl.
> >
> > This feature is mutually exclusive with virtio 1.0. Even if the vhost device
> > advertises virtio 1.0 and legacy cross-endian features, it cannot receive
> > aknowledgement for both at the same time.
> >
> > Hot paths are being preserved from any penalty when the config option is
> > disabled or when virtio 1.0 is being used.
> >
> > Signed-off-by: Greg Kurz <gkurz@xxxxxxxxxxxxxxxxxx>
> > ---
> > drivers/vhost/Kconfig | 10 ++++++++++
> > drivers/vhost/net.c | 5 +++++
> > drivers/vhost/scsi.c | 4 ++++
> > drivers/vhost/test.c | 4 ++++
> > drivers/vhost/vhost.c | 19 +++++++++++++++++++
> > drivers/vhost/vhost.h | 13 ++++++++++++-
> > include/uapi/linux/vhost.h | 10 ++++++++++
> > 7 files changed, 64 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/vhost/Kconfig b/drivers/vhost/Kconfig
> > index 017a1e8..5bb8da9 100644
> > --- a/drivers/vhost/Kconfig
> > +++ b/drivers/vhost/Kconfig
> > @@ -32,3 +32,13 @@ config VHOST
> > ---help---
> > This option is selected by any driver which needs to access
> > the core of vhost.
> > +
> > +config VHOST_SET_ENDIAN_LEGACY
> > + bool "Cross-endian support for host kernel accelerator"
> > + default n
> > + ---help---
> > + This option allows vhost to support guests with a different byte
> > + ordering
>
> from host
>
> >. It is disabled by default since it adds overhead and it
> > + is only needed by a few platforms (powerpc and arm).
> > +
> > + If unsure, say "n".
>
> "N"
>
> > diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> > index 2bbfc25..5274a44 100644
> > --- a/drivers/vhost/net.c
> > +++ b/drivers/vhost/net.c
> > @@ -1011,6 +1011,11 @@ static int vhost_net_set_features(struct vhost_net *n, u64 features)
> > vhost_hlen = 0;
> > sock_hlen = hdr_len;
> > }
> > +
> > + if (features & ((1ULL << VHOST_F_SET_ENDIAN_LEGACY) |
> > + (1ULL << VIRTIO_F_VERSION_1)))
> > + return -EINVAL;
> > +
> > mutex_lock(&n->dev.mutex);
> > if ((features & (1 << VHOST_F_LOG_ALL)) &&
> > !vhost_log_access_ok(&n->dev)) {
> > diff --git a/drivers/vhost/scsi.c b/drivers/vhost/scsi.c
> > index 71df240..b53e9c2 100644
> > --- a/drivers/vhost/scsi.c
> > +++ b/drivers/vhost/scsi.c
> > @@ -1544,6 +1544,10 @@ static int vhost_scsi_set_features(struct vhost_scsi *vs, u64 features)
> > if (features & ~VHOST_SCSI_FEATURES)
> > return -EOPNOTSUPP;
> >
> > + if (features & ((1ULL << VHOST_F_SET_ENDIAN_LEGACY) |
> > + (1ULL << VIRTIO_F_VERSION_1)))
> > + return -EINVAL;
> > +
> > mutex_lock(&vs->dev.mutex);
> > if ((features & (1 << VHOST_F_LOG_ALL)) &&
> > !vhost_log_access_ok(&vs->dev)) {
> > diff --git a/drivers/vhost/test.c b/drivers/vhost/test.c
> > index d9c501e..cfefdad 100644
> > --- a/drivers/vhost/test.c
> > +++ b/drivers/vhost/test.c
> > @@ -243,6 +243,10 @@ static int vhost_test_set_features(struct vhost_test *n, u64 features)
> > {
> > struct vhost_virtqueue *vq;
> >
> > + if (features & ((1ULL << VHOST_F_SET_ENDIAN_LEGACY) |
> > + (1ULL << VIRTIO_F_VERSION_1)))
> > + return -EINVAL;
> > +
> > mutex_lock(&n->dev.mutex);
> > if ((features & (1 << VHOST_F_LOG_ALL)) &&
> > !vhost_log_access_ok(&n->dev)) {
>
> Above seems to prevent users from specifying either
> VIRTIO_F_VERSION_1 or VHOST_F_SET_ENDIAN_LEGACY.
> Does it actually work?
>

Arggg no it doesn't *of course*... I've added these bogus checks lately
and didn't re-test :(

> > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> > index 2ee2826..60a0f32 100644
> > --- a/drivers/vhost/vhost.c
> > +++ b/drivers/vhost/vhost.c
> > @@ -199,6 +199,7 @@ static void vhost_vq_reset(struct vhost_dev *dev,
> > vq->call = NULL;
> > vq->log_ctx = NULL;
> > vq->memory = NULL;
> > + vq->legacy_big_endian = false;
> > }
> >
> > static int vhost_worker(void *data)
> > @@ -806,6 +807,24 @@ long vhost_vring_ioctl(struct vhost_dev *d, int ioctl, void __user *argp)
> > } else
> > filep = eventfp;
> > break;
> > +#ifdef CONFIG_VHOST_SET_ENDIAN_LEGACY
> > + case VHOST_SET_VRING_ENDIAN_LEGACY:
> > + {
> > + struct vhost_vring_endian e;
> > +
> > + if (!vhost_has_feature(vq, VHOST_F_SET_ENDIAN_LEGACY)) {
> > + r = -EINVAL;
> > + break;
> > + }
> > +
> > + if (copy_from_user(&e, argp, sizeof(e))) {
> > + r = -EFAULT;
> > + break;
> > + }
> > + vq->legacy_big_endian = e.is_big_endian;
> > + break;
> > + }
> > +#endif
> > default:
> > r = -ENOIOCTLCMD;
> > }
> > diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
> > index 4e9a186..d50881d 100644
> > --- a/drivers/vhost/vhost.h
> > +++ b/drivers/vhost/vhost.h
> > @@ -106,6 +106,9 @@ struct vhost_virtqueue {
> > /* Log write descriptors */
> > void __user *log_base;
> > struct vhost_log *log;
> > +
> > + /* We need to know the device endianness with legacy virtio. */
> > + bool legacy_big_endian;
> > };
> >
> > struct vhost_dev {
> > @@ -165,7 +168,11 @@ enum {
> > VHOST_FEATURES = (1ULL << VIRTIO_F_NOTIFY_ON_EMPTY) |
> > (1ULL << VIRTIO_RING_F_INDIRECT_DESC) |
> > (1ULL << VIRTIO_RING_F_EVENT_IDX) |
> > - (1ULL << VHOST_F_LOG_ALL),
> > + (1ULL << VHOST_F_LOG_ALL) |
> > +#ifdef CONFIG_VHOST_SET_ENDIAN_LEGACY
> > + (1ULL << VHOST_F_SET_ENDIAN_LEGACY) |
> > +#endif
> > + 0ULL,
> > };
> >
> > static inline bool vhost_has_feature(struct vhost_virtqueue *vq, int bit)
> > @@ -177,6 +184,10 @@ static inline bool vhost_is_little_endian(struct vhost_virtqueue *vq)
> > {
> > if (vhost_has_feature(vq, VIRTIO_F_VERSION_1))
> > return true;
> > +#ifdef CONFIG_VHOST_SET_ENDIAN_LEGACY
> > + if (vhost_has_feature(vq, VHOST_F_SET_ENDIAN_LEGACY))
> > + return !vq->legacy_big_endian;
>
> why do we need to check the feature bit?
> How about simple
> return !vq->legacy_big_endian;
> here?

This is a runtime feature. Even for powerpc, cross-endian won't be the
most common scenario. Userspace may have cleared the bit if it doesn't
need/want it.

> All you need to do is set legacy_big_endian to
> !virtio_legacy_is_little_endian() on reset.
> Maybe rename to legacy_is_little_endian so you don't
> need to reverse the logic.
>

Do you mean vhost doesn't need userspace to provide the endianness
to be used ? Please elaborate on the meaning of "reset" here... I
am confused.

> > +#endif
> > return virtio_legacy_is_little_endian();
> > }
> >
> > diff --git a/include/uapi/linux/vhost.h b/include/uapi/linux/vhost.h
> > index bb6a5b4..09d2a48 100644
> > --- a/include/uapi/linux/vhost.h
> > +++ b/include/uapi/linux/vhost.h
> > @@ -47,6 +47,11 @@ struct vhost_vring_addr {
> > __u64 log_guest_addr;
> > };
> >
> > +struct vhost_vring_endian {
> > + unsigned int index;
> > + bool is_big_endian;
>
> bool in uapi is a bad idea.
> Generally, I think you can use vhost_vring_state here.
>

Ok to turn is_big_endian into an int but why hijacking vhost_vring_state ?

> > +};
> > +
> > struct vhost_memory_region {
> > __u64 guest_phys_addr;
> > __u64 memory_size; /* bytes */
> > @@ -103,6 +108,9 @@ struct vhost_memory {
> > /* Get accessor: reads index, writes value in num */
> > #define VHOST_GET_VRING_BASE _IOWR(VHOST_VIRTIO, 0x12, struct vhost_vring_state)
> >
> > +/* Set endianness for the ring (legacy virtio only) */
> > +#define VHOST_SET_VRING_ENDIAN_LEGACY _IOW(VHOST_VIRTIO, 0x13, struct vhost_vring_endian)
> > +
> > /* The following ioctls use eventfd file descriptors to signal and poll
> > * for events. */
> >
>
> You also need a GET ioctl, as a matter of policy.
>

Ok. Will add.

> > @@ -126,6 +134,8 @@ struct vhost_memory {
> > #define VHOST_F_LOG_ALL 26
> > /* vhost-net should add virtio_net_hdr for RX, and strip for TX packets. */
> > #define VHOST_NET_F_VIRTIO_NET_HDR 27
> > +/* the vring endianness can be explicitly set (legacy virtio only). */
> > +#define VHOST_F_SET_ENDIAN_LEGACY 28
> >
> > /* VHOST_SCSI specific definitions */
>
>
> VHOST_F_SET_ENDIAN_LEGACY doesn't seem too useful.
> Is this so userspace can detect kernel configuration?

Yes. Also, it is quite easy to "un-ack" the feature when
it is not wanted.

> I think probing VHOST_SET_VRING_ENDIAN_LEGACY should
> be sufficient for this.
>

I'll get rid of the feature.

Thanks for your time Michael.

--
Greg

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