Re: [PATCH RESEND] xenbus: Add support for xenbus backend in stub domain

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

On Tue, May 08, 2012 at 09:46:57AM -0400, Daniel De Graaf wrote:
> Add an ioctl to the /dev/xen/xenbus_backend device allowing the xenbus
> backend to be started after the kernel has booted. This allows xenstore
> to run in a different domain from the dom0.
> 
> Signed-off-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>
> ---
>  drivers/xen/xenbus/xenbus_comms.c       |    6 ++++
>  drivers/xen/xenbus/xenbus_comms.h       |    1 +
>  drivers/xen/xenbus/xenbus_dev_backend.c |   51 +++++++++++++++++++++++++++++++
>  include/xen/grant_table.h               |    2 +
>  include/xen/xenbus_dev.h                |    3 ++
>  5 files changed, 63 insertions(+), 0 deletions(-)
> 
> diff --git a/drivers/xen/xenbus/xenbus_comms.c b/drivers/xen/xenbus/xenbus_comms.c
> index 2eff7a6..52fe7ad 100644
> --- a/drivers/xen/xenbus/xenbus_comms.c
> +++ b/drivers/xen/xenbus/xenbus_comms.c
> @@ -234,3 +234,9 @@ int xb_init_comms(void)
>  
>  	return 0;
>  }
> +
> +void xb_deinit_comms(void)
> +{
> +	unbind_from_irqhandler(xenbus_irq, &xb_waitq);
> +	xenbus_irq = 0;
> +}
> diff --git a/drivers/xen/xenbus/xenbus_comms.h b/drivers/xen/xenbus/xenbus_comms.h
> index 6e42800..c8abd3b 100644
> --- a/drivers/xen/xenbus/xenbus_comms.h
> +++ b/drivers/xen/xenbus/xenbus_comms.h
> @@ -35,6 +35,7 @@
>  
>  int xs_init(void);
>  int xb_init_comms(void);
> +void xb_deinit_comms(void);
>  
>  /* Low level routines. */
>  int xb_write(const void *data, unsigned len);
> diff --git a/drivers/xen/xenbus/xenbus_dev_backend.c b/drivers/xen/xenbus/xenbus_dev_backend.c
> index 3d3be78..be738c4 100644
> --- a/drivers/xen/xenbus/xenbus_dev_backend.c
> +++ b/drivers/xen/xenbus/xenbus_dev_backend.c
> @@ -8,7 +8,11 @@
>  
>  #include <xen/xen.h>
>  #include <xen/page.h>
> +#include <xen/xenbus.h>
>  #include <xen/xenbus_dev.h>
> +#include <xen/grant_table.h>
> +#include <xen/events.h>
> +#include <asm/xen/hypervisor.h>
>  
>  #include "xenbus_comms.h"
>  
> @@ -22,6 +26,50 @@ static int xenbus_backend_open(struct inode *inode, struct file *filp)
>  	return nonseekable_open(inode, filp);
>  }
>  
> +static long xenbus_alloc(domid_t domid)
> +{
> +	struct evtchn_alloc_unbound arg;
> +	int err = -EEXIST;
> +
> +	xs_suspend();
> +
> +	/* If xenstored_ready is nonzero, that means we have already talked to
> +	 * xenstore and set up watches. These watches will be restored by
> +	 * xs_resume, but that requires communication over the port established
> +	 * below that is not visible to anyone until the ioctl returns.
> +	 *
> +	 * This can be resolved by splitting the ioctl into two parts
> +	 * (postponing the resume until xenstored is active) but this is
> +	 * unnecessarily complex for the intended use where xenstored is only
> +	 * started once - so return -EEXIST if it's already running.
> +	 */
> +	if (xenstored_ready)
> +		goto out_err;
> +
> +	gnttab_grant_foreign_access_ref(GNTTAB_RESERVED_XENSTORE, domid,
> +			virt_to_mfn(xen_store_interface), 0 /* writable */);
> +
> +	arg.dom = DOMID_SELF;
> +	arg.remote_dom = domid;

If I specify as domid = DOMID_SELF what will happen? Should we filter some
of the obvious no-no values? Like DOMID_IO, DOMID_SELF?
> +
> +	err = HYPERVISOR_event_channel_op(EVTCHNOP_alloc_unbound, &arg);
> +	if (err)
> +		goto out_err;
> +
> +	if (xen_store_evtchn > 0)
> +		xb_deinit_comms();
> +
> +	xen_store_evtchn = arg.port;
> +
> +	xs_resume();
> +
> +	return arg.port;
> +
> + out_err:
> +	xs_suspend_cancel();
> +	return err;
> +}
> +
>  static long xenbus_backend_ioctl(struct file *file, unsigned int cmd, unsigned long data)
>  {
>  	if (!capable(CAP_SYS_ADMIN))
> @@ -33,6 +81,9 @@ static long xenbus_backend_ioctl(struct file *file, unsigned int cmd, unsigned l
>  				return xen_store_evtchn;
>  			return -ENODEV;
>  
> +		case IOCTL_XENBUS_BACKEND_SETUP:
> +			return xenbus_alloc(data);
> +
>  		default:
>  			return -ENOTTY;
>  	}
> diff --git a/include/xen/grant_table.h b/include/xen/grant_table.h
> index 15f8a00..11e27c3 100644
> --- a/include/xen/grant_table.h
> +++ b/include/xen/grant_table.h
> @@ -46,6 +46,8 @@
>  
>  #include <xen/features.h>
>  
> +#define GNTTAB_RESERVED_XENSTORE 1
> +
>  /* NR_GRANT_FRAMES must be less than or equal to that configured in Xen */
>  #define NR_GRANT_FRAMES 4
>  
> diff --git a/include/xen/xenbus_dev.h b/include/xen/xenbus_dev.h
> index ac5f0fe..bbee8c6 100644
> --- a/include/xen/xenbus_dev.h
> +++ b/include/xen/xenbus_dev.h
> @@ -38,4 +38,7 @@
>  #define IOCTL_XENBUS_BACKEND_EVTCHN			\
>  	_IOC(_IOC_NONE, 'B', 0, 0)
>  
> +#define IOCTL_XENBUS_BACKEND_SETUP			\
> +	_IOC(_IOC_NONE, 'B', 1, 0)
> +
>  #endif /* __LINUX_XEN_XENBUS_DEV_H__ */
> -- 
> 1.7.7.6