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[Xen-devel] [PATCH RFC] xen: arm: Log a warning message when a deprecated hypercall is used

A few folks have been caught out by OSes which call e.g.
HYPERVISOR_event_channel_op_compat which has been deprecated since
3.2.2 (i.e. long before Xen on ARM). Existing x86 code can still
safely and quietly using those calls, waiting for an unsuspecting ARM
porter to turn up and trip over it. This turns out to be rather
perplexing when it happens, since it can be obscured e.g. by various
conditionals like __XEN_INTERFACE_VERSION__ what is actually being

Note that I'm making a distinction here between hypercalls which are
simply not used/implemented on arm (yet) and those which were
deprecated and replaced by a newer variant prior to Xen on ARM even
being invented.  The latter will never be implemented on ARM and have
non-deprecated aliases leading to confusion so those are the ones for
which a warning is useful.

Signed-off-by: Ian Campbell <ian.campbell@xxxxxxxxxx>
Cc: Jan Beulich <JBeulich@xxxxxxxx>
Cc: Keir Fraser <keir@xxxxxxx>
Cc: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
Cc: Anthony PERARD <anthony.perard@xxxxxxxxxx>
RFC since I'm not sure how extreme our reaction ought to be here, e.g.
I considered domain_crash() or even panic() when in a debug build. A
XENLOG_DEBUG message is about the most benign of the options.

Jan/Keir, although this is ARM specific I'd welcome your views as
x86/REST maintainers.

Ard, I've not actually run this -- any chance you could re-b0rk your
Tianocore image and give it a go?
 xen/arch/arm/traps.c |   30 ++++++++++++++++++++++++++++++
 1 file changed, 30 insertions(+)

diff --git a/xen/arch/arm/traps.c b/xen/arch/arm/traps.c
index ad046e8..89cbde6 100644
--- a/xen/arch/arm/traps.c
+++ b/xen/arch/arm/traps.c
@@ -1148,6 +1148,22 @@ die:
+static register_t do_deprecated_hypercall(void)
+    struct cpu_user_regs *regs = guest_cpu_user_regs();
+    const register_t op =
+#ifdef CONFIG_ARM_64
+        !is_32bit_domain(current->domain) ?
+            regs->x16
+        :
+            regs->r12;
+    gdprintk(XENLOG_DEBUG, "%pv: deprecated hypercall %ld\n",
+             current, (unsigned long)op);
+    return -ENOSYS;
 typedef register_t (*arm_hypercall_fn_t)(
     register_t, register_t, register_t, register_t, register_t);
@@ -1167,15 +1183,29 @@ typedef struct {
         .fn = (arm_hypercall_fn_t) &do_arm_ ## _name,                \
         .nr_args = _nr_args,                                         \
+ * Only use this for hypercalls which were deprecated (i.e. replaced
+ * by something else) before Xen on ARM was created, i.e. *not* for
+ * hypercalls which are simply not yet used on ARM.
+ */
+#define HYPERCALL_DEPRECATED(_name, _nr_args)                   \
+    [ __HYPERVISOR_##_name ] = {                                \
+        .fn = (arm_hypercall_fn_t) &do_deprecated_hypercall,    \
+        .nr_args = _nr_args,                                    \
+    }
 static arm_hypercall_t arm_hypercall_table[] = {
     HYPERCALL(memory_op, 2),
     HYPERCALL(domctl, 1),
     HYPERCALL(sched_op, 2),
+    HYPERCALL_DEPRECATED(sched_op_compat, 2),
     HYPERCALL(console_io, 3),
     HYPERCALL(xen_version, 2),
     HYPERCALL(xsm_op, 1),
     HYPERCALL(event_channel_op, 2),
+    HYPERCALL_DEPRECATED(event_channel_op_compat, 1),
     HYPERCALL(physdev_op, 2),
+    HYPERCALL_DEPRECATED(physdev_op_compat, 1),
     HYPERCALL(sysctl, 2),
     HYPERCALL(hvm_op, 2),
     HYPERCALL(grant_table_op, 3),

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