Re: [PATCH RFC] vhost: add ioctl to query nregions upper limit

Igor Mammedov
2015-06-24T14:07:37Z

On Wed, 24 Jun 2015 15:49:27 +0200
"Michael S. Tsirkin" <mst@redhat.com> wrote:

> Userspace currently simply tries to give vhost as many regions
> as it happens to have, but you only have the mem table
> when you have initialized a large part of VM, so graceful
> failure is very hard to support.
>
> The result is that userspace tends to fail catastrophically.
>
> Instead, add a new ioctl so userspace can find out how much kernel
> supports, up front. This returns a positive value that we commit to.
>
> Also, document our contract with legacy userspace: when running on an
> old kernel, you get -1 and you can assume at least 64 slots. Since 0
> value's left unused, let's make that mean that the current userspace
> behaviour (trial and error) is required, just in case we want it back.
>
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> Cc: Igor Mammedov <imammedo@redhat.com>
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> ---
> include/uapi/linux/vhost.h | 17 ++++++++++++++++-
> drivers/vhost/vhost.c | 5 +++++
> 2 files changed, 21 insertions(+), 1 deletion(-)
>
> diff --git a/include/uapi/linux/vhost.h b/include/uapi/linux/vhost.h
> index ab373191..f71fa6d 100644
> --- a/include/uapi/linux/vhost.h
> +++ b/include/uapi/linux/vhost.h
> @@ -80,7 +80,7 @@ struct vhost_memory {
> * Allows subsequent call to VHOST_OWNER_SET to succeed. */
> #define VHOST_RESET_OWNER _IO(VHOST_VIRTIO, 0x02)
>
> -/* Set up/modify memory layout */
> +/* Set up/modify memory layout: see also VHOST_GET_MEM_MAX_NREGIONS below. */
> #define VHOST_SET_MEM_TABLE _IOW(VHOST_VIRTIO, 0x03, struct vhost_memory)
>
> /* Write logging setup. */
> @@ -127,6 +127,21 @@ struct vhost_memory {
> /* Set eventfd to signal an error */
> #define VHOST_SET_VRING_ERR _IOW(VHOST_VIRTIO, 0x22, struct vhost_vring_file)
>
> +/* Query upper limit on nregions in VHOST_SET_MEM_TABLE arguments.
> + * Returns:
> + * 0 < value <= MAX_INT - gives the upper limit, higher values will fail
> + * 0 - there's no static limit: try and see if it works
> + * -1 - on failure
> + */
> +#define VHOST_GET_MEM_MAX_NREGIONS _IO(VHOST_VIRTIO, 0x23)
> +
> +/* Returned by VHOST_GET_MEM_MAX_NREGIONS to mean there's no static limit:
> + * try and it'll work if you are lucky. */
> +#define VHOST_MEM_MAX_NREGIONS_NONE 0
is it needed? we always have a limit,
or don't have IOCTL => -1 => old try and see way

> +/* We support at least as many nregions in VHOST_SET_MEM_TABLE:
> + * for use on legacy kernels without VHOST_GET_MEM_MAX_NREGIONS support. */
> +#define VHOST_MEM_MAX_NREGIONS_DEFAULT 64
^^^ not used below,
if it's for legacy then perhaps s/DEFAULT/LEGACY/

> +
> /* VHOST_NET specific defines */
>
> /* Attach virtio net ring to a raw socket, or tap device.
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index 9e8e004..3b68f9d 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -917,6 +917,11 @@ long vhost_dev_ioctl(struct vhost_dev *d, unsigned int ioctl, void __user *argp)
> long r;
> int i, fd;
>
> + if (ioctl == VHOST_GET_MEM_MAX_NREGIONS) {
> + r = VHOST_MEMORY_MAX_NREGIONS;
> + goto done;
> + }
> +
> /* If you are not the owner, you can become one */
> if (ioctl == VHOST_SET_OWNER) {
> r = vhost_dev_set_owner(d);