[PATCH v2] eal/x86: cache queried CPU flags

[Bruce Richardson]

Rather than re-querying the HW each time a CPU flag is requested, we can
just save the return value in the flags array. This should speed up
repeated querying of CPU flags, and provides a workaround for a reported
issue where errors are seen with constant querying of the AVX-512 CPU
flag from a non-AVX VM.

Bugzilla ID: 1501

Signed-off-by: Bruce Richardson <bruce.richard...@intel.com>

---
V2: Add compiler barrier to prevent issues with multi-threaded calls
---
 lib/eal/x86/rte_cpuflags.c | 22 +++++++++++++++++-----
 1 file changed, 17 insertions(+), 5 deletions(-)

diff --git a/lib/eal/x86/rte_cpuflags.c b/lib/eal/x86/rte_cpuflags.c
index 26163ab746..90389c66fc 100644
--- a/lib/eal/x86/rte_cpuflags.c
+++ b/lib/eal/x86/rte_cpuflags.c
@@ -8,8 +8,10 @@
 #include <errno.h>
 #include <stdint.h>
 #include <string.h>
+#include <stdbool.h>
 
 #include "rte_cpuid.h"
+#include "rte_atomic.h"
 
 /**
  * Struct to hold a processor feature entry
@@ -21,12 +23,14 @@ struct feature_entry {
        uint32_t bit;                           /**< cpuid register bit */
 #define CPU_FLAG_NAME_MAX_LEN 64
        char name[CPU_FLAG_NAME_MAX_LEN];       /**< String for printing */
+       bool has_value;
+       bool value;
 };
 
 #define FEAT_DEF(name, leaf, subleaf, reg, bit) \
        [RTE_CPUFLAG_##name] = {leaf, subleaf, reg, bit, #name },
 
-const struct feature_entry rte_cpu_feature_table[] = {
+struct feature_entry rte_cpu_feature_table[] = {
        FEAT_DEF(SSE3, 0x00000001, 0, RTE_REG_ECX,  0)
        FEAT_DEF(PCLMULQDQ, 0x00000001, 0, RTE_REG_ECX,  1)
        FEAT_DEF(DTES64, 0x00000001, 0, RTE_REG_ECX,  2)
@@ -147,7 +151,7 @@ const struct feature_entry rte_cpu_feature_table[] = {
 int
 rte_cpu_get_flag_enabled(enum rte_cpu_flag_t feature)
 {
-       const struct feature_entry *feat;
+       struct feature_entry *feat;
        cpuid_registers_t regs;
        unsigned int maxleaf;
 
@@ -156,6 +160,8 @@ rte_cpu_get_flag_enabled(enum rte_cpu_flag_t feature)
                return -ENOENT;
 
        feat = &rte_cpu_feature_table[feature];
+       if (feat->has_value)
+               return feat->value;
 
        if (!feat->leaf)
                /* This entry in the table wasn't filled out! */
@@ -163,8 +169,10 @@ rte_cpu_get_flag_enabled(enum rte_cpu_flag_t feature)
 
        maxleaf = __get_cpuid_max(feat->leaf & 0x80000000, NULL);
 
-       if (maxleaf < feat->leaf)
-               return 0;
+       if (maxleaf < feat->leaf) {
+               feat->value = 0;
+               goto out;
+       }
 
 #ifdef RTE_TOOLCHAIN_MSVC
        __cpuidex(regs, feat->leaf, feat->subleaf);
@@ -175,7 +183,11 @@ rte_cpu_get_flag_enabled(enum rte_cpu_flag_t feature)
 #endif
 
        /* check if the feature is enabled */
-       return (regs[feat->reg] >> feat->bit) & 1;
+       feat->value = (regs[feat->reg] >> feat->bit) & 1;
+out:
+       rte_compiler_barrier();
+       feat->has_value = true;
+       return feat->value;
 }
 
 const char *
-- 
2.43.0