[PATCH v2 4/4] x86: short-circuit certain cpu_has_* when x86-64-v{2,3} are in effect

[Jan Beulich <jbeulich@xxxxxxxx>]

Certain fallback code can be made subject to DCE this way. Note that
CX16 has no compiler provided manifest constant, so CONFIG_* are used
there instead. Note also that we don't have cpu_has_movbe nor
cpu_has_lzcnt (aka cpu_has_abm).

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
Of course we could use IS_ENABLED(CONFIG_X86_64_V<n>) everywhere, but as
CX16 shows this isn't necessarily better than the #if/#else approach
based on compiler-provided manifest symbols. While not really intended
to be used that way, it looks as if we could also use
IS_ENABLED(__POPCNT__) and alike if we thought this would end up neater
(because of avoiding the #ifdef).

We could go further and also short-circuit SSE*, AVX and alike, which we
don't use outside of the emulator. This then would of course call for
also having a way to select x86-64-v4.
---
v2: Also cover XSAVE.

--- a/xen/arch/x86/include/asm/cpufeature.h
+++ b/xen/arch/x86/include/asm/cpufeature.h
@@ -76,15 +76,25 @@ static inline bool boot_cpu_has(unsigned
 #define cpu_has_eist            boot_cpu_has(X86_FEATURE_EIST)
 #define cpu_has_ssse3           boot_cpu_has(X86_FEATURE_SSSE3)
 #define cpu_has_fma             boot_cpu_has(X86_FEATURE_FMA)
-#define cpu_has_cx16            boot_cpu_has(X86_FEATURE_CX16)
+#define cpu_has_cx16            (IS_ENABLED(CONFIG_X86_64_V2) || \
+                                 IS_ENABLED(CONFIG_X86_64_V3) || \
+                                 boot_cpu_has(X86_FEATURE_CX16))
 #define cpu_has_pdcm            boot_cpu_has(X86_FEATURE_PDCM)
 #define cpu_has_pcid            boot_cpu_has(X86_FEATURE_PCID)
 #define cpu_has_sse4_1          boot_cpu_has(X86_FEATURE_SSE4_1)
 #define cpu_has_sse4_2          boot_cpu_has(X86_FEATURE_SSE4_2)
 #define cpu_has_x2apic          boot_cpu_has(X86_FEATURE_X2APIC)
+#ifdef __POPCNT__
+#define cpu_has_popcnt          true
+#else
 #define cpu_has_popcnt          boot_cpu_has(X86_FEATURE_POPCNT)
+#endif
 #define cpu_has_aesni           boot_cpu_has(X86_FEATURE_AESNI)
+#ifdef __XSAVE__
+#define cpu_has_xsave           true
+#else
 #define cpu_has_xsave           boot_cpu_has(X86_FEATURE_XSAVE)
+#endif
 #define cpu_has_avx             boot_cpu_has(X86_FEATURE_AVX)
 #define cpu_has_f16c            boot_cpu_has(X86_FEATURE_F16C)
 #define cpu_has_rdrand          boot_cpu_has(X86_FEATURE_RDRAND)
@@ -114,11 +124,19 @@ static inline bool boot_cpu_has(unsigned
 #define cpu_has_xsaves          boot_cpu_has(X86_FEATURE_XSAVES)
 
 /* CPUID level 0x00000007:0.ebx */
+#ifdef __BMI__
+#define cpu_has_bmi1            true
+#else
 #define cpu_has_bmi1            boot_cpu_has(X86_FEATURE_BMI1)
+#endif
 #define cpu_has_hle             boot_cpu_has(X86_FEATURE_HLE)
 #define cpu_has_avx2            boot_cpu_has(X86_FEATURE_AVX2)
 #define cpu_has_smep            boot_cpu_has(X86_FEATURE_SMEP)
+#ifdef __BMI2__
+#define cpu_has_bmi2            true
+#else
 #define cpu_has_bmi2            boot_cpu_has(X86_FEATURE_BMI2)
+#endif
 #define cpu_has_invpcid         boot_cpu_has(X86_FEATURE_INVPCID)
 #define cpu_has_rtm             boot_cpu_has(X86_FEATURE_RTM)
 #define cpu_has_pqe             boot_cpu_has(X86_FEATURE_PQE)