tools/libxc: retry hypercall in case of EFAULT

A hypercall issued via the privcmd driver can very rarely return
-EFAULT even if the hypercall buffers are locked in memory. This
happens for hypercall buffers in user memory when the Linux kernel
is doing memory scans e.g. for page migration or compaction.

Retry the getpageframeinfo3 hypercall up to 2 times in case
-EFAULT is returned and the hypervisor might see invalid PTEs for
user hypercall buffers (which should be the case only if the kernel
doesn't offer a /dev/xen/hypercall node).

Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>

tools/libxc/xc_private.c

@@ -224,7 +224,7 @@ int xc_get_pfn_type_batch(xc_interface *xch, uint32_t dom,
     domctl.domain = dom;
     domctl.u.getpageframeinfo3.num = num;
     set_xen_guest_handle(domctl.u.getpageframeinfo3.array, arr);
-    rc = do_domctl(xch, &domctl);
+    rc = do_domctl_retry_efault(xch, &domctl);
     xc_hypercall_bounce_post(xch, arr);
     return rc;
 }

tools/libxc/xc_private.h

@@ -254,9 +254,13 @@ out1:
     return ret;
 }
 
-static inline int do_domctl(xc_interface *xch, struct xen_domctl *domctl)
+static inline int do_domctl_maybe_retry_efault(xc_interface *xch,
+                                               struct xen_domctl *domctl,
+                                               unsigned int retries)
 {
     int ret = -1;
+    unsigned int retry_cnt = 0;
+
     DECLARE_HYPERCALL_BOUNCE(domctl, sizeof(*domctl), XC_HYPERCALL_BUFFER_BOUNCE_BOTH);
 
     domctl->interface_version = XEN_DOMCTL_INTERFACE_VERSION;
@@ -267,8 +271,11 @@ static inline int do_domctl(xc_interface *xch, struct xen_domctl *domctl)
         goto out1;
     }
 
-    ret = xencall1(xch->xcall, __HYPERVISOR_domctl,
-                   HYPERCALL_BUFFER_AS_ARG(domctl));
+    do {
+        ret = xencall1(xch->xcall, __HYPERVISOR_domctl,
+                       HYPERCALL_BUFFER_AS_ARG(domctl));
+    } while ( ret < 0 && errno == EFAULT && retry_cnt++ < retries );
+
     if ( ret < 0 )
     {
         if ( errno == EACCES )
@@ -281,6 +288,18 @@ static inline int do_domctl(xc_interface *xch, struct xen_domctl *domctl)
     return ret;
 }
 
+static inline int do_domctl(xc_interface *xch, struct xen_domctl *domctl)
+{
+    return do_domctl_maybe_retry_efault(xch, domctl, 0);
+}
+
+static inline int do_domctl_retry_efault(xc_interface *xch, struct xen_domctl *domctl)
+{
+    unsigned int retries = xencall_buffers_never_fault(xch->xcall) ? 0 : 2;
+
+    return do_domctl_maybe_retry_efault(xch, domctl, retries);
+}
+
 static inline int do_sysctl(xc_interface *xch, struct xen_sysctl *sysctl)
 {
     int ret = -1;