[xen master] x86/hvm: only register the r/o subpage ops when needed

[patchbot@xxxxxxx]

commit 06fd6818eae5878397e36d542251e1ad17805262
Author:     Roger Pau Monne <roger.pau@xxxxxxxxxx>
AuthorDate: Fri Apr 11 10:31:22 2025 +0200
Commit:     Roger Pau Monne <roger.pau@xxxxxxxxxx>
CommitDate: Tue Apr 29 14:49:54 2025 +0200

    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@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
---
 xen/arch/x86/hvm/hvm.c        |  3 ++-
 xen/arch/x86/include/asm/mm.h |  1 +
 xen/arch/x86/mm.c             | 16 ++++++++++++++++
 3 files changed, 19 insertions(+), 1 deletion(-)

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;
 
--
generated by git-patchbot for /home/xen/git/xen.git#master