[Xen-devel] [[PATCH v3] 2/4] xen/arm: cpufeature: Add helper to check constant caps

[Julien Grall <julien.grall@xxxxxxx>]

Some capababilities are set right during boot and will never change
afterwards. At the moment, the function cpu_have_caps will check whether
the cap is enabled from the memory.

It is possible to avoid the load from the memory by using an
ALTERNATIVE. With that the check is just reduced to 1 instruction.

Signed-off-by: Julien Grall <julien.grall@xxxxxxx>

---

This is the static key for the poor. At some point we might want to
introduce something similar to static key in Xen.

    Changes in v2:
        - Use unlikely
---
 xen/include/asm-arm/cpufeature.h | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/xen/include/asm-arm/cpufeature.h b/xen/include/asm-arm/cpufeature.h
index 3de6b54301..c6cbc2ec84 100644
--- a/xen/include/asm-arm/cpufeature.h
+++ b/xen/include/asm-arm/cpufeature.h
@@ -63,6 +63,18 @@ static inline bool cpus_have_cap(unsigned int num)
     return test_bit(num, cpu_hwcaps);
 }
 
+/* System capability check for constant cap */
+#define cpus_have_const_cap(num) ({                 \
+        bool __ret;                                 \
+                                                    \
+        asm volatile (ALTERNATIVE("mov %0, #0",     \
+                                  "mov %0, #1",     \
+                                  num)              \
+                      : "=r" (__ret));              \
+                                                    \
+        unlikely(__ret);                            \
+        })
+
 static inline void cpus_set_cap(unsigned int num)
 {
     if (num >= ARM_NCAPS)
-- 
2.11.0


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