[PATCH 4/5] x86/hvm: only register the r/o subpage ops when needed

[Roger Pau Monne <roger.pau@xxxxxxxxxx>]

MMIO operation handlers can be expensive to process, hence attempt to
register only those that will be needed by the domain.

Subpage r/o MMIO regions are added exclusively at boot, further limit their
addition to strictly before the initial domain gets created, so by the time
initial domain creation happens Xen knows whether subpage is required or
not.  This allows only registering the MMIO handler when there are
subpage regions to handle.

Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
Could possibly be part of the previous patch, but strictly speaking is an
improvement, as even before the previous patch subpage r/o was always
called even when no subpage regions are registered.
---
 xen/arch/x86/hvm/hvm.c        |  5 +++--
 xen/arch/x86/include/asm/mm.h |  1 +
 xen/arch/x86/mm.c             | 16 ++++++++++++++++
 3 files changed, 20 insertions(+), 2 deletions(-)

diff --git a/xen/arch/x86/hvm/hvm.c b/xen/arch/x86/hvm/hvm.c
index 21f005b0947c..1a5dfc07813d 100644
--- a/xen/arch/x86/hvm/hvm.c
+++ b/xen/arch/x86/hvm/hvm.c
@@ -764,8 +764,9 @@ int hvm_domain_initialise(struct domain *d,
 
     register_portio_handler(d, XEN_HVM_DEBUGCONS_IOPORT, 1, hvm_print_line);
 
-    /* Handler for r/o MMIO subpage accesses. */
-    register_mmio_handler(d, &subpage_mmio_ops);
+    if ( subpage_ro_active() )
+        /* Handler for r/o MMIO subpage accesses. */
+        register_mmio_handler(d, &subpage_mmio_ops);
 
     if ( hvm_tsc_scaling_supported )
         d->arch.hvm.tsc_scaling_ratio = hvm_default_tsc_scaling_ratio;
diff --git a/xen/arch/x86/include/asm/mm.h b/xen/arch/x86/include/asm/mm.h
index c2e9ef6e5023..aeb8ebcf2d56 100644
--- a/xen/arch/x86/include/asm/mm.h
+++ b/xen/arch/x86/include/asm/mm.h
@@ -561,6 +561,7 @@ struct subpage_ro_range {
     void __iomem *mapped;
     DECLARE_BITMAP(ro_elems, PAGE_SIZE / MMIO_RO_SUBPAGE_GRAN);
 };
+bool subpage_ro_active(void);
 struct subpage_ro_range *subpage_mmio_find_page(mfn_t mfn);
 void __iomem *subpage_mmio_map_page(struct subpage_ro_range *entry);
 void subpage_mmio_write_emulate(
diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
index f59c7816fba5..3bc6304d831c 100644
--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -4922,6 +4922,11 @@ long arch_memory_op(unsigned long cmd, 
XEN_GUEST_HANDLE_PARAM(void) arg)
     return rc;
 }
 
+bool subpage_ro_active(void)
+{
+    return !list_empty(&subpage_ro_ranges);
+}
+
 struct subpage_ro_range *subpage_mmio_find_page(mfn_t mfn)
 {
     struct subpage_ro_range *entry;
@@ -5011,6 +5016,17 @@ int __init subpage_mmio_ro_add(
          !IS_ALIGNED(size, MMIO_RO_SUBPAGE_GRAN) )
         return -EINVAL;
 
+    /*
+     * Force all r/o subregions to be registered before initial domain
+     * creation, so that the emulation handlers can be added only when there
+     * are pages registered.
+     */
+    if ( system_state >= SYS_STATE_smp_boot )
+    {
+        ASSERT_UNREACHABLE();
+        return -EILSEQ;
+    }
+
     if ( !size )
         return 0;
 
-- 
2.48.1