3 files changed, 78 insertions, 0 deletions

diff --git a/hw/i386/kvm/xen_overlay.c b/hw/i386/kvm/xen_overlay.c
index a2441e2b4e..8685d87959 100644
--- a/hw/i386/kvm/xen_overlay.c
+++ b/hw/i386/kvm/xen_overlay.c
@@ -44,6 +44,7 @@ struct XenOverlayState {
     MemoryRegion shinfo_mem;
     void *shinfo_ptr;
     uint64_t shinfo_gpa;
+    bool long_mode;
 };
 
 struct XenOverlayState *xen_overlay_singleton;
@@ -96,9 +97,21 @@ static void xen_overlay_realize(DeviceState *dev, Error **errp)
 
     s->shinfo_ptr = memory_region_get_ram_ptr(&s->shinfo_mem);
     s->shinfo_gpa = INVALID_GPA;
+    s->long_mode = false;
     memset(s->shinfo_ptr, 0, XEN_PAGE_SIZE);
 }
 
+static int xen_overlay_pre_save(void *opaque)
+{
+    /*
+     * Fetch the kernel's idea of long_mode to avoid the race condition
+     * where the guest has set the hypercall page up in 64-bit mode but
+     * not yet made a hypercall by the time migration happens, so qemu
+     * hasn't yet noticed.
+     */
+    return xen_sync_long_mode();
+}
+
 static int xen_overlay_post_load(void *opaque, int version_id)
 {
     XenOverlayState *s = opaque;
@@ -107,6 +120,9 @@ static int xen_overlay_post_load(void *opaque, int version_id)
         xen_overlay_do_map_page(&s->shinfo_mem, s->shinfo_gpa);
         xen_overlay_set_be_shinfo(s->shinfo_gpa >> XEN_PAGE_SHIFT);
     }
+    if (s->long_mode) {
+        xen_set_long_mode(true);
+    }
 
     return 0;
 }
@@ -121,9 +137,11 @@ static const VMStateDescription xen_overlay_vmstate = {
     .version_id = 1,
     .minimum_version_id = 1,
     .needed = xen_overlay_is_needed,
+    .pre_save = xen_overlay_pre_save,
     .post_load = xen_overlay_post_load,
     .fields = (VMStateField[]) {
         VMSTATE_UINT64(shinfo_gpa, XenOverlayState),
+        VMSTATE_BOOL(long_mode, XenOverlayState),
         VMSTATE_END_OF_LIST()
     }
 };
@@ -208,3 +226,47 @@ void *xen_overlay_get_shinfo_ptr(void)
 
     return s->shinfo_ptr;
 }
+
+int xen_sync_long_mode(void)
+{
+    int ret;
+    struct kvm_xen_hvm_attr xa = {
+        .type = KVM_XEN_ATTR_TYPE_LONG_MODE,
+    };
+
+    if (!xen_overlay_singleton) {
+        return -ENOENT;
+    }
+
+    ret = kvm_vm_ioctl(kvm_state, KVM_XEN_HVM_GET_ATTR, &xa);
+    if (!ret) {
+        xen_overlay_singleton->long_mode = xa.u.long_mode;
+    }
+
+    return ret;
+}
+
+int xen_set_long_mode(bool long_mode)
+{
+    int ret;
+    struct kvm_xen_hvm_attr xa = {
+        .type = KVM_XEN_ATTR_TYPE_LONG_MODE,
+        .u.long_mode = long_mode,
+    };
+
+    if (!xen_overlay_singleton) {
+        return -ENOENT;
+    }
+
+    ret = kvm_vm_ioctl(kvm_state, KVM_XEN_HVM_SET_ATTR, &xa);
+    if (!ret) {
+        xen_overlay_singleton->long_mode = xa.u.long_mode;
+    }
+
+    return ret;
+}
+
+bool xen_is_long_mode(void)
+{
+    return xen_overlay_singleton && xen_overlay_singleton->long_mode;
+}
diff --git a/hw/i386/kvm/xen_overlay.h b/hw/i386/kvm/xen_overlay.h
index 00cff05bb0..5c46a0b036 100644
--- a/hw/i386/kvm/xen_overlay.h
+++ b/hw/i386/kvm/xen_overlay.h
@@ -17,4 +17,8 @@ void xen_overlay_create(void);
 int xen_overlay_map_shinfo_page(uint64_t gpa);
 void *xen_overlay_get_shinfo_ptr(void);
 
+int xen_sync_long_mode(void);
+int xen_set_long_mode(bool long_mode);
+bool xen_is_long_mode(void);
+
 #endif /* QEMU_XEN_OVERLAY_H */
diff --git a/target/i386/kvm/xen-emu.c b/target/i386/kvm/xen-emu.c
index ebea27caf6..be6d85f2cb 100644
--- a/target/i386/kvm/xen-emu.c
+++ b/target/i386/kvm/xen-emu.c
@@ -20,6 +20,8 @@
 #include "trace.h"
 #include "sysemu/runstate.h"
 
+#include "hw/i386/kvm/xen_overlay.h"
+
 #include "hw/xen/interface/version.h"
 #include "hw/xen/interface/sched.h"
 
@@ -282,6 +284,16 @@ int kvm_xen_handle_exit(X86CPU *cpu, struct kvm_xen_exit *exit)
         return -1;
     }
 
+    /*
+     * The kernel latches the guest 32/64 mode when the MSR is used to fill
+     * the hypercall page. So if we see a hypercall in a mode that doesn't
+     * match our own idea of the guest mode, fetch the kernel's idea of the
+     * "long mode" to remain in sync.
+     */
+    if (exit->u.hcall.longmode != xen_is_long_mode()) {
+        xen_sync_long_mode();
+    }
+
     if (!do_kvm_xen_handle_exit(cpu, exit)) {
         /*
          * Some hypercalls will be deliberately "implemented" by returning