The alternative call infrastructure is x86-only for now, but the common iommu
code has a variant and more common code wants to use the infrastructure.

Introduce CONFIG_ALTERNATIVE_CALL and a conditional implemetnation so common
code can use the optimisation when available, without requiring all
architectures to implement no-op stubs.

Write some documentation, which was thus far entirely absent, covering the
requirements for an architecture to implement this optimsiation, and how to
use the infrastructure in general code.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
---
CC: Jan Beulich <JBeulich@suse.com>
CC: Roger Pau Monné <roger.pau@citrix.com>
CC: Wei Liu <wl@xen.org>
CC: Stefano Stabellini <sstabellini@kernel.org>
CC: Julien Grall <julien@xen.org>
CC: Volodymyr Babchuk <Volodymyr_Babchuk@epam.com>
CC: Bob Eshleman <bobbyeshleman@gmail.com>
CC: Alistair Francis <alistair.francis@wdc.com>
CC: Connor Davis <connojdavis@gmail.com>
CC: Daniel P. Smith <dpsmith@apertussolutions.com>

This is a pre-requisite to "xsm: refactor xsm_ops handling" to avoid breaking
the ARM build.

Build test for the XSM code:

  diff --git a/xen/xsm/xsm_core.c b/xen/xsm/xsm_core.c
  index 5eab21e1b168..592074e8f41c 100644
  --- a/xen/xsm/xsm_core.c
  +++ b/xen/xsm/xsm_core.c
  @@ -195,6 +195,16 @@ bool __init has_xsm_magic(paddr_t start)
   }
    #endif

  +#include <xen/alternative-call.h>
  +struct foo {
  +    int (*bar)(void *);
  +} foo __alt_call_maybe_initdata;
  +
  +int test_alternative_call(void)
  +{
  +    return alternative_call(foo.bar, NULL);
  +}
  +
   int __init register_xsm(struct xsm_operations *ops)
    {
         if ( verify(ops) )
---
 xen/arch/x86/Kconfig               |  1 +
 xen/common/Kconfig                 |  3 ++
 xen/include/xen/alternative-call.h | 65 ++++++++++++++++++++++++++++++++++++++
 3 files changed, 69 insertions(+)
 create mode 100644 xen/include/xen/alternative-call.h

diff --git a/xen/arch/x86/Kconfig b/xen/arch/x86/Kconfig
index 9b164db64187..c91cdd83dc8a 100644
--- a/xen/arch/x86/Kconfig
+++ b/xen/arch/x86/Kconfig
@@ -9,6 +9,7 @@ config X86
 	select ARCH_SUPPORTS_INT128
 	select CORE_PARKING
 	select HAS_ALTERNATIVE
+	select ALTERNATIVE_CALL
 	select HAS_COMPAT
 	select HAS_CPUFREQ
 	select HAS_EHCI
diff --git a/xen/common/Kconfig b/xen/common/Kconfig
index 0ddd18e11af3..1594ce4e7313 100644
--- a/xen/common/Kconfig
+++ b/xen/common/Kconfig
@@ -25,6 +25,9 @@ config GRANT_TABLE
 config HAS_ALTERNATIVE
 	bool
 
+config ALTERNATIVE_CALL
+	bool
+
 config HAS_COMPAT
 	bool
 
diff --git a/xen/include/xen/alternative-call.h b/xen/include/xen/alternative-call.h
new file mode 100644
index 000000000000..11d1c2606818
--- /dev/null
+++ b/xen/include/xen/alternative-call.h
@@ -0,0 +1,65 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef XEN_ALTERNATIVE_CALL
+#define XEN_ALTERNATIVE_CALL
+
+/*
+ * Some subsystems in Xen may have multiple implementions, which can be
+ * resolved to a single implementation at boot time.  By default, this will
+ * result in the use of function pointers.
+ *
+ * Some architectures may have mechanisms for dynamically modifying .text.
+ * Using this mechnaism, function pointers can be converted to direct calls
+ * which are typically more efficient at runtime.
+ *
+ * For architectures to support:
+ *
+ * - Implement alternative_{,v}call() in asm/alternative.h.  Code generation
+ *   requirements are to emit a function pointer call at build time, and stash
+ *   enough metadata to simplify the call at boot once the implementation has
+ *   been resolved.
+ * - Select ALTERNATIVE_CALL in Kconfig.
+ *
+ * To use:
+ *
+ * Consider the following simplified example.
+ *
+ *  1) struct foo_ops __alt_call_maybe_initdata ops;
+ *
+ *  2) struct foo_ops __alt_call_maybe_initconst foo_a_ops = { ... };
+ *     struct foo_ops __alt_call_maybe_initconst foo_b_ops = { ... };
+ *
+ *     void foo_init(void)
+ *     {
+ *         ...
+ *         if ( use_impl_a )
+ *             ops = *foo_a_ops;
+ *         else if ( use_impl_b )
+ *             ops = *foo_b_ops;
+ *         ...
+ *     }
+ *
+ *  3) alternative_call(ops.bar, ...);
+ *
+ * There needs to a single ops object (1) which will eventually contain the
+ * function pointers.  This should be populated in foo's init() function (2)
+ * by one of the available implementations.  To call functions, use
+ * alternative_{,v}call() referencing the main ops object (3).
+ */
+
+#ifdef CONFIG_ALTERNATIVE_CALL
+
+#include <asm/alternative.h>
+
+#define __alt_call_maybe_initdata  __initdata
+#define __alt_call_maybe_initconst __initconst
+
+#else
+
+#define alternative_call(func, args...)  (func)(args)
+#define alternative_vcall(func, args...) (func)(args)
+
+#define __alt_call_maybe_initdata
+#define __alt_call_maybe_initconst
+
+#endif /* !CONFIG_ALTERNATIVE_CALL */
+#endif /* XEN_ALTERNATIVE_CALL */
-- 
2.11.0