[xen master] x86/domain: adjust limitation on shared_info allocation below 4G

[patchbot@xxxxxxx]

commit 8a9255405ca60b58d69ebb1a2dcdeb23cc42df1f
Author:     Roger Pau Monne <roger.pau@xxxxxxxxxx>
AuthorDate: Tue Feb 3 10:50:48 2026 +0100
Commit:     Roger Pau Monne <roger.pau@xxxxxxxxxx>
CommitDate: Thu Feb 5 21:27:40 2026 +0100

    x86/domain: adjust limitation on shared_info allocation below 4G
    
    The limitation of shared_info being allocated below 4G to fit in the
    start_info field only applies to 32bit PV guests.  On 64bit PV guests the
    start_info field is 64bits wide.  HVM guests don't use start_info at all.
    
    Drop the restriction in arch_domain_create() and instead free and
    re-allocate the page from memory below 4G if needed in switch_compat(),
    when the guest is set to run in 32bit PV mode.
    
    Fixes: 3cadc0469d5c ("x86_64: shared_info must be allocated below 4GB as it 
is advertised to 32-bit guests via a 32-bit machine address field in 
start_info.")
    Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
    Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
 xen/arch/x86/domain.c    |  6 +-----
 xen/arch/x86/pv/domain.c | 28 ++++++++++++++++++++++++++++
 xen/common/domain.c      |  2 +-
 xen/include/xen/domain.h |  2 ++
 4 files changed, 32 insertions(+), 6 deletions(-)

diff --git a/xen/arch/x86/domain.c b/xen/arch/x86/domain.c
index edb76366b5..8b2f33f1a0 100644
--- a/xen/arch/x86/domain.c
+++ b/xen/arch/x86/domain.c
@@ -881,11 +881,7 @@ int arch_domain_create(struct domain *d,
     if ( d->arch.ioport_caps == NULL )
         goto fail;
 
-    /*
-     * The shared_info machine address must fit in a 32-bit field within a
-     * 32-bit guest's start_info structure. Hence we specify MEMF_bits(32).
-     */
-    if ( (d->shared_info = alloc_xenheap_pages(0, MEMF_bits(32))) == NULL )
+    if ( (d->shared_info = alloc_xenheap_page()) == NULL )
         goto fail;
 
     clear_page(d->shared_info);
diff --git a/xen/arch/x86/pv/domain.c b/xen/arch/x86/pv/domain.c
index 01499582d2..d16583a745 100644
--- a/xen/arch/x86/pv/domain.c
+++ b/xen/arch/x86/pv/domain.c
@@ -247,6 +247,34 @@ int switch_compat(struct domain *d)
     d->arch.has_32bit_shinfo = 1;
     d->arch.pv.is_32bit = true;
 
+    /*
+     * For 32bit PV guests the shared_info machine address must fit in a 32-bit
+     * field within the guest's start_info structure.  We might need to free
+     * the current page and allocate a new one that fulfills this requirement.
+     */
+    if ( virt_to_maddr(d->shared_info) >> 32 )
+    {
+        shared_info_t *prev = d->shared_info;
+
+        d->shared_info = alloc_xenheap_pages(0, MEMF_bits(32));
+        if ( !d->shared_info )
+        {
+            d->shared_info = prev;
+            rc = -ENOMEM;
+            goto undo_and_fail;
+        }
+        clear_page(d->shared_info);
+        share_xen_page_with_guest(virt_to_page(d->shared_info), d, SHARE_rw);
+        /*
+         * Ensure all pointers to the old shared_info page are replaced.  vCPUs
+         * below XEN_LEGACY_MAX_VCPUS may have stashed a pointer to
+         * shared_info->vcpu_info[id].
+         */
+        for_each_vcpu ( d, v )
+            vcpu_info_reset(v);
+        put_page(virt_to_page(prev));
+    }
+
     for_each_vcpu( d, v )
     {
         if ( (rc = setup_compat_arg_xlat(v)) ||
diff --git a/xen/common/domain.c b/xen/common/domain.c
index 3fd907e274..de6fdf5923 100644
--- a/xen/common/domain.c
+++ b/xen/common/domain.c
@@ -307,7 +307,7 @@ static void vcpu_check_shutdown(struct vcpu *v)
     spin_unlock(&d->shutdown_lock);
 }
 
-static void vcpu_info_reset(struct vcpu *v)
+void vcpu_info_reset(struct vcpu *v)
 {
     struct domain *d = v->domain;
 
diff --git a/xen/include/xen/domain.h b/xen/include/xen/domain.h
index 627ca0e071..93c0fd00c1 100644
--- a/xen/include/xen/domain.h
+++ b/xen/include/xen/domain.h
@@ -83,6 +83,8 @@ void cf_check free_pirq_struct(void *ptr);
 int  arch_vcpu_create(struct vcpu *v);
 void arch_vcpu_destroy(struct vcpu *v);
 
+void vcpu_info_reset(struct vcpu *v);
+
 int map_guest_area(struct vcpu *v, paddr_t gaddr, unsigned int size,
                    struct guest_area *area,
                    void (*populate)(void *dst, struct vcpu *v));
--
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