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Message-ID: <20150407155228.GA1907@redhat.com>
Date: Tue, 7 Apr 2015 17:52:28 +0200
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Greg Kurz <gkurz@...ux.vnet.ibm.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, 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>
EINVAL probably makes more sense?
> ---
> 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;
EINVAL probably makes more sense? But I'm not sure this
is helpful: one can set VIRTIO_F_VERSION_1 afterwards,
and your patch does not seem to detect this.
> +
> + 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;
> +}
Should be -ENOIOCTLCMD?
> +#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;
> }
I think we also want to forbid this with a running backend.
> 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 */
I'd prefer 0 and 1, return EINVAL on other values.
> +#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. */
>
--
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