i386/xen: manage and save/restore Xen guest long_mode setting

Xen will "latch" the guest's 32-bit or 64-bit ("long mode") setting when
the guest writes the MSR to fill in the hypercall page, or when the guest
sets the event channel callback in HVM_PARAM_CALLBACK_IRQ.

KVM handles the former and sets the kernel's long_mode flag accordingly.
The latter will be handled in userspace. Keep them in sync by noticing
when a hypercall is made in a mode that doesn't match qemu's idea of
the guest mode, and resyncing from the kernel. Do that same sync right
before serialization too, in case the guest has set the hypercall page
but hasn't yet made a system call.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Reviewed-by: Paul Durrant <paul@xen.org>

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;
+}

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 */

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