Re: [PATCH 1/2] vhost: add ioctl to query nregions upper limit

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Jul 29, 2015 at 04:29:22PM +0200, Igor Mammedov wrote:
> From: "Michael S. Tsirkin" <mst@redhat.com>
> 
> 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.

What's wrong with reading the module parameter value? It's there in
sysfs ...

> 
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> Signed-off-by: Igor Mammedov <imammedo@redhat.com>
> ---
>  drivers/vhost/vhost.c      |  7 ++++++-
>  include/uapi/linux/vhost.h | 17 ++++++++++++++++-
>  2 files changed, 22 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index eec2f11..76dc0cf 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -30,7 +30,7 @@
>  
>  #include "vhost.h"
>  
> -static ushort max_mem_regions = 64;
> +static ushort max_mem_regions = VHOST_MEM_MAX_NREGIONS_DEFAULT;
>  module_param(max_mem_regions, ushort, 0444);
>  MODULE_PARM_DESC(max_mem_regions,
>  	"Maximum number of memory regions in memory map. (default: 64)");
> @@ -944,6 +944,11 @@ 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 = max_mem_regions;
> +		goto done;
> +	}
> +
>  	/* If you are not the owner, you can become one */
>  	if (ioctl == VHOST_SET_OWNER) {
>  		r = vhost_dev_set_owner(d);
> diff --git a/include/uapi/linux/vhost.h b/include/uapi/linux/vhost.h
> index ab373191..2511954 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
> +/* 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
> +
>  /* VHOST_NET specific defines */
>  
>  /* Attach virtio net ring to a raw socket, or tap device.
> -- 
> 1.8.3.1