x86/domctl: have XEN_DOMCTL_getpageframeinfo3 preemptible

This hypercall can take a long time to finish because it attempts to
grab the `hostp2m' lock up to 1024 times. The accumulated wait for the
lock can take several seconds.

This can easily happen with a guest with 32 vcpus and plenty of RAM,
during localhost migration.

While the patch doesn't fix the problem with the lock contention and
the fact that the `hostp2m' lock is currently global (and not on a
single page), it is still an improvement to the hypercall. It will in
particular, down the road, allow dropping the arbitrary limit of 1024
entries per request.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
master commit: 48599114d3ca24157c25f6684bb9322f6dca12bb
master date: 2019-11-26 14:16:09 +0100

diff --git a/xen/arch/x86/domctl.c b/xen/arch/x86/domctl.c
index 0a31f0001641a24ed457ff820cc84b0b291bb8de..204928cda082d95a383ce46fb7fb3d21fdfa0331 100644 (file)
--- a/xen/arch/x86/domctl.c
+++ b/xen/arch/x86/domctl.c
@@ -495,6 +495,26 @@ long arch_do_domctl(
                 ret = -EFAULT;
                 break;
             }
+
+            /*
+             * Avoid checking for preemption when the `hostp2m' lock isn't
+             * involve, i.e. non-translated guest, and avoid preemption on
+             * the last iteration.
+             */
+            if ( paging_mode_translate(d) &&
+                 likely((i + 1) < num) && hypercall_preempt_check() )
+            {
+                domctl->u.getpageframeinfo3.num = num - i - 1;
+                domctl->u.getpageframeinfo3.array.p =
+                    guest_handle + ((i + 1) * width);
+                if ( __copy_to_guest(u_domctl, domctl, 1) )
+                {
+                    ret = -EFAULT;
+                    break;
+                }
+                return hypercall_create_continuation(__HYPERVISOR_domctl,
+                                                     "h", u_domctl);
+            }
         }
 
         break;

diff --git a/xen/include/public/domctl.h b/xen/include/public/domctl.h
index 7e1cf210758994aa91c3e41365c06022262ede2e..6f9be8166ee7057c11df9b77aa5290962f31f681 100644 (file)
--- a/xen/include/public/domctl.h
+++ b/xen/include/public/domctl.h
@@ -148,6 +148,10 @@ DEFINE_XEN_GUEST_HANDLE(xen_domctl_getdomaininfo_t);
 #define XEN_DOMCTL_PFINFO_LTAB_MASK (0xfU<<28)
 
 /* XEN_DOMCTL_getpageframeinfo3 */
+/*
+ * Both value `num' and `array' may get modified by the hypercall to allow
+ * preemption.
+ */
 struct xen_domctl_getpageframeinfo3 {
     /* IN variables. */
     uint64_aligned_t num;