[Xen-devel] [XEN PATCH for-4.13 v2] x86/domctl: have XEN_DOMCTL_getpageframeinfo3 preemptible

[Anthony PERARD <anthony.perard@xxxxxxxxxx>]

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.

Signed-off-by: Anthony PERARD <anthony.perard@xxxxxxxxxx>
---

Notes:
    Changes in v2:
    - fix coding style.
    - check for translated guests.
    - avoid preemption on the last iteration.
    - add a comment in the public header.
    
    Further possible improvement to the hypercall:
    - process several GFNs after grabbing the hostp2m lock
    - Remove the limit

 xen/arch/x86/domctl.c       | 20 ++++++++++++++++++++
 xen/include/public/domctl.h |  4 ++++
 2 files changed, 24 insertions(+)

diff --git a/xen/arch/x86/domctl.c b/xen/arch/x86/domctl.c
index 43e368d63bb9..b461aadbd640 100644
--- a/xen/arch/x86/domctl.c
+++ b/xen/arch/x86/domctl.c
@@ -425,6 +425,26 @@ 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 a03e80e5984a..1b69eb75cb20 100644
--- a/xen/include/public/domctl.h
+++ b/xen/include/public/domctl.h
@@ -163,6 +163,10 @@ DEFINE_XEN_GUEST_HANDLE(xen_domctl_getdomaininfo_t);
 #define XEN_DOMCTL_PFINFO_LTAB_MASK (0xfU<<28)
 
 /* XEN_DOMCTL_getpageframeinfo3 */
+/*
+ * Both value `num' and `array' are modified by the hypercall to allow
+ * preemption.
+ */
 struct xen_domctl_getpageframeinfo3 {
     /* IN variables. */
     uint64_aligned_t num;
-- 
Anthony PERARD


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