From: Roger Pau Monne <roger.pau@citrix.com>
Date: Fri, 11 Apr 2025 08:31:22 +0000 (+0200)
Subject: x86/hvm: only register the r/o subpage ops when needed
X-Git-Url: http://xenbits.xensource.com/gitweb?a=commitdiff_plain;h=06fd6818eae5878397e36d542251e1ad17805262;p=xen.git

x86/hvm: only register the r/o subpage ops when needed

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@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
---

diff --git a/xen/arch/x86/hvm/hvm.c b/xen/arch/x86/hvm/hvm.c
index 6b998387e3..4cb2e13046 100644
--- a/xen/arch/x86/hvm/hvm.c
+++ b/xen/arch/x86/hvm/hvm.c
@@ -692,7 +692,8 @@ int hvm_domain_initialise(struct domain *d,
 
     register_portio_handler(d, XEN_HVM_DEBUGCONS_IOPORT, 1, hvm_print_line);
 
-    register_subpage_ro_handler(d);
+    if ( subpage_ro_active() )
+        register_subpage_ro_handler(d);
 
     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 c2e9ef6e50..aeb8ebcf2d 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 7ead2db3cb..6697984507 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;