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Message-ID: <20150408102534.1baddf7b@bahia.local>
Date: Wed, 8 Apr 2015 10:25:34 +0200
From: Greg Kurz <gkurz@...ux.vnet.ibm.com>
To: "Michael S. Tsirkin" <mst@...hat.com>
Cc: Rusty Russell <rusty@...tcorp.com.au>, linux-api@...r.kernel.org,
linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH v3 7/7] vhost: feature to set the vring endianness
On Tue, 7 Apr 2015 18:11:29 +0200
"Michael S. Tsirkin" <mst@...hat.com> wrote:
> On Tue, Apr 07, 2015 at 02:19:31PM +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).
> >
> > The ioctls introduced by this patch are for legacy only: virtio 1.0
> > devices are returned EPERM.
> >
> > Signed-off-by: Greg Kurz <gkurz@...ux.vnet.ibm.com>
> > ---
> > drivers/vhost/Kconfig | 10 ++++++++
> > drivers/vhost/vhost.c | 55 ++++++++++++++++++++++++++++++++++++++++++++
> > drivers/vhost/vhost.h | 17 +++++++++++++-
> > include/uapi/linux/vhost.h | 5 ++++
> > 4 files changed, 86 insertions(+), 1 deletion(-)
> >
> > Changes since v2:
> > - fixed typos in Kconfig description
> > - renamed vq->legacy_big_endian to vq->legacy_is_little_endian
> > - vq->legacy_is_little_endian reset to default in vhost_vq_reset()
> > - dropped VHOST_F_SET_ENDIAN_LEGACY feature
> > - dropped struct vhost_vring_endian from the user API (re-use
> > struct vhost_vring_state instead)
> > - added VHOST_GET_VRING_ENDIAN_LEGACY ioctl
> > - introduced more helpers and stubs to avoid polluting the code with ifdefs
> >
> >
> > diff --git a/drivers/vhost/Kconfig b/drivers/vhost/Kconfig
> > index 017a1e8..0aec88c 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".
> > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> > index 2ee2826..3529a3c 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_is_little_endian = virtio_legacy_is_little_endian();
> > }
> >
> > static int vhost_worker(void *data)
> > @@ -630,6 +631,54 @@ static long vhost_set_memory(struct vhost_dev *d, struct vhost_memory __user *m)
> > return 0;
> > }
> >
> > +#ifdef CONFIG_VHOST_SET_ENDIAN_LEGACY
> > +static long vhost_set_vring_endian_legacy(struct vhost_virtqueue *vq,
> > + void __user *argp)
> > +{
> > + struct vhost_vring_state s;
> > +
> > + if (vhost_has_feature(vq, VIRTIO_F_VERSION_1))
> > + return -EPERM;
> > +
> > + if (copy_from_user(&s, argp, sizeof(s)))
> > + return -EFAULT;
> > +
> > + vq->legacy_is_little_endian = !!s.num;
> > + return 0;
> > +}
> > +
> > +static long vhost_get_vring_endian_legacy(struct vhost_virtqueue *vq,
> > + u32 idx,
> > + void __user *argp)
> > +{
> > + struct vhost_vring_state s = {
> > + .index = idx,
> > + .num = vq->legacy_is_little_endian
> > + };
> > +
> > + if (vhost_has_feature(vq, VIRTIO_F_VERSION_1))
> > + return -EPERM;
> > +
> > + if (copy_to_user(argp, &s, sizeof(s)))
> > + return -EFAULT;
> > +
> > + return 0;
> > +}
> > +#else
> > +static long vhost_set_vring_endian_legacy(struct vhost_virtqueue *vq,
> > + void __user *argp)
> > +{
> > + return 0;
> > +}
> > +
> > +static long vhost_get_vring_endian_legacy(struct vhost_virtqueue *vq,
> > + u32 idx,
> > + void __user *argp)
> > +{
> > + return 0;
> > +}
> > +#endif /* CONFIG_VHOST_SET_ENDIAN_LEGACY */
> > +
> > long vhost_vring_ioctl(struct vhost_dev *d, int ioctl, void __user *argp)
> > {
> > struct file *eventfp, *filep = NULL;
> > @@ -806,6 +855,12 @@ long vhost_vring_ioctl(struct vhost_dev *d, int ioctl, void __user *argp)
> > } else
> > filep = eventfp;
> > break;
> > + case VHOST_SET_VRING_ENDIAN_LEGACY:
> > + r = vhost_set_vring_endian_legacy(vq, argp);
> > + break;
> > + case VHOST_GET_VRING_ENDIAN_LEGACY:
> > + r = vhost_get_vring_endian_legacy(vq, idx, argp);
> > + break;
> > default:
> > r = -ENOIOCTLCMD;
> > }
> > diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
> > index 4e9a186..981ba06 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_is_little_endian;
> > };
> >
> > struct vhost_dev {
> > @@ -173,11 +176,23 @@ static inline bool vhost_has_feature(struct vhost_virtqueue *vq, int bit)
> > return vq->acked_features & (1ULL << bit);
> > }
> >
> > +#ifdef CONFIG_VHOST_SET_ENDIAN_LEGACY
> > +static inline bool vhost_legacy_is_little_endian(struct vhost_virtqueue *vq)
> > +{
> > + return vq->legacy_is_little_endian;
> > +}
> > +#else
> > +static inline bool vhost_legacy_is_little_endian(struct vhost_virtqueue *vq)
> > +{
> > + return virtio_legacy_is_little_endian();
> > +}
> > +#endif
> > +
> > static inline bool vhost_is_little_endian(struct vhost_virtqueue *vq)
> > {
> > if (vhost_has_feature(vq, VIRTIO_F_VERSION_1))
> > return true;
> > - return virtio_legacy_is_little_endian();
> > + return vhost_legacy_is_little_endian(vq);
> > }
> >
> > /* Memory accessors */
> > diff --git a/include/uapi/linux/vhost.h b/include/uapi/linux/vhost.h
> > index bb6a5b4..1b01a72 100644
> > --- a/include/uapi/linux/vhost.h
> > +++ b/include/uapi/linux/vhost.h
> > @@ -103,6 +103,11 @@ 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) */
> > +/* num is 0 for big endian, other values mean little endian */
> > +#define VHOST_SET_VRING_ENDIAN_LEGACY _IOW(VHOST_VIRTIO, 0x13, struct vhost_vring_state)
> > +#define VHOST_GET_VRING_ENDIAN_LEGACY _IOW(VHOST_VIRTIO, 0x14, struct vhost_vring_state)
> > +
> > /* The following ioctls use eventfd file descriptors to signal and poll
> > * for events. */
> >
>
> So if the config feature is enabled, you actually check two
> things: 1. feature 2. ioctl
>
> Why not override is_le when we set VIRTIO_F_VERSION_1?
> I guess you are worried that setting a value and not being
> able to read it back is ugly. We can add a flag to track
> it though. So here's an idea
> I would probably rename to G/SET_BIG_ENDIAN then it's obvious
> it's legacy. And just document that it's ignored with
> VIRTIO_F_VERSION_1.
>
> make sure it's not changed when ring is running
>
> simply set vq->is_le correctly when we start/stop the device
> vq->is_le = vhost_has_feature(vq, VIRTIO_F_VERSION_1) ||
> !vq->user_be;
>
> static inline bool vhost_is_little_endian(struct vhost_virtqueue *vq)
> {
> return vq->is_le;
> }
>
>
I like the idea. I'll give it a try.
Thanks for your help Michael.
--
Greg
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