[RFC PATCH 3/6] xen/arm: Add ARM64_HAS_LSE_ATOMICS hwcap

[Ash Wilding <ash.j.wilding@xxxxxxxxx>]

This patch introduces the ARM64_HAS_LSE_ATOMICS hwcap.

While doing this, CONFIG_ARM64_LSE_ATOMICS is added to control whether
the hwcap is actually detected and set at runtime. Without this Kconfig
being set we will always fallback on LL/SC atomics using Armv8.0 exlusive
accesses.

Note this patch does not actually add the ALTERNATIVE() switching based
on the hwcap being detected and set; that comes later in the series.

Signed-off-by: Ash Wilding <ash.j.wilding@xxxxxxxxx>
---
 xen/arch/arm/Kconfig             | 11 +++++++++++
 xen/arch/arm/Makefile            |  1 +
 xen/arch/arm/lse.c               | 13 +++++++++++++
 xen/include/asm-arm/cpufeature.h |  3 ++-
 4 files changed, 27 insertions(+), 1 deletion(-)
 create mode 100644 xen/arch/arm/lse.c

diff --git a/xen/arch/arm/Kconfig b/xen/arch/arm/Kconfig
index 2777388265..febc41e492 100644
--- a/xen/arch/arm/Kconfig
+++ b/xen/arch/arm/Kconfig
@@ -78,6 +78,17 @@ config SBSA_VUART_CONSOLE
          Allows a guest to use SBSA Generic UART as a console. The
          SBSA Generic UART implements a subset of ARM PL011 UART.
 
+config ARM64_LSE_ATOMICS
+       bool "Armv8.1-LSE Atomics"
+       depends on ARM_64 && HAS_ALTERNATIVE
+       default y
+       ---help---
+       When set, dynamically patch Xen at runtime to use Armv8.1-LSE
+       atomics when supported by the system.
+
+       When unset, or when Armv8.1-LSE atomics are not supported by the
+       system, fallback on LL/SC atomics using Armv8.0 exclusive accesses.
+
 config ARM_SSBD
        bool "Speculative Store Bypass Disable" if EXPERT
        depends on HAS_ALTERNATIVE
diff --git a/xen/arch/arm/Makefile b/xen/arch/arm/Makefile
index 296c5e68bb..cadd0ad253 100644
--- a/xen/arch/arm/Makefile
+++ b/xen/arch/arm/Makefile
@@ -63,6 +63,7 @@ obj-y += vsmc.o
 obj-y += vpsci.o
 obj-y += vuart.o
 extra-y += $(TARGET_SUBARCH)/head.o
+obj-$(CONFIG_ARM64_LSE_ATOMICS) += lse.o
 
 #obj-bin-y += ....o
 
diff --git a/xen/arch/arm/lse.c b/xen/arch/arm/lse.c
new file mode 100644
index 0000000000..8274dac671
--- /dev/null
+++ b/xen/arch/arm/lse.c
@@ -0,0 +1,13 @@
+
+#include <asm/cpufeature.h>
+#include <xen/init.h>
+
+static int __init update_lse_caps(void)
+{
+    if ( cpu_has_lse_atomics )
+        cpus_set_cap(ARM64_HAS_LSE_ATOMICS);
+
+    return 0;
+}
+
+__initcall(update_lse_caps);
diff --git a/xen/include/asm-arm/cpufeature.h b/xen/include/asm-arm/cpufeature.h
index 2366926e82..48c172ee29 100644
--- a/xen/include/asm-arm/cpufeature.h
+++ b/xen/include/asm-arm/cpufeature.h
@@ -42,8 +42,9 @@
 #define ARM_SSBD 7
 #define ARM_SMCCC_1_1 8
 #define ARM64_WORKAROUND_AT_SPECULATE 9
+#define ARM64_HAS_LSE_ATOMICS 10
 
-#define ARM_NCAPS           10
+#define ARM_NCAPS           11
 
 #ifndef __ASSEMBLY__
 
-- 
2.24.3 (Apple Git-128)