[Xen-devel] [PATCH] xenbus: Add support for xenbus backend in stub domain

[Daniel De Graaf <dgdegra@xxxxxxxxxxxxx>]

This adds an ioctl to the /dev/xen/xenbus_backend device allowing the
xenbus backend to be started after the kernel has booted. This is
intended to allow dom0 to start another domain to run xenstore.

Signed-off-by: Daniel De Graaf <dgdegra@xxxxxxxxxxxxx>
---
 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;
+
+       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__ */
-- 
1.7.7.5


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