@@ -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
@@ -25,6 +25,9 @@ config GRANT_TABLE
config HAS_ALTERNATIVE
bool
+config ALTERNATIVE_CALL
+ bool
+
config HAS_COMPAT
bool
new file mode 100644
@@ -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 */
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