x86: highmem handling assistance hypercalls

While looking at the origin of very frequently executed hypercalls I
realized that the high page accessor functions in Linux would be good
candidates to handle in the hypervisor - clearing or copying to/from
a high page is a pretty frequent operation (provided there's enough
memory in the domain). While prior to the first submission I only
measured kernel builds (where the results are not hinting at a
meaningful improvement), I now found time to do a more specific
analysis: page clearing is being improved by about 20%, page copying
doesn't seem to significantly benefit (though that may be an effect of
the simplistic copy_page() implementation Xen currently uses) -
nevertheless I would think that if one function is supported by the
hypervisor, then the other should also be.

Signed-off-by: Jan Beulich <jbeulich@novell.com>

1 files changed, 2 insertions, 1 deletions

diff --git a/xen/common/kernel.c b/xen/common/kernel.c
index 2a88602e51..f3af91f2ed 100644
--- a/xen/common/kernel.c
+++ b/xen/common/kernel.c
@@ -221,7 +221,8 @@ DO(xen_version)(int cmd, XEN_GUEST_HANDLE(void) arg)
                 fi.submap |= 1U << XENFEAT_supervisor_mode_kernel;
 #ifdef CONFIG_X86
             if ( !is_hvm_vcpu(current) )
-                fi.submap |= 1U << XENFEAT_mmu_pt_update_preserve_ad;
+                fi.submap |= (1U << XENFEAT_mmu_pt_update_preserve_ad) |
+                             (1U << XENFEAT_highmem_assist);
 #endif
             break;
         default: