[PATCH v4 3/3] 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), and we no longer have
cpu_has_{popcnt,bmi1,bmi2}.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
Reviewed-by: Jason Andryuk <jandryuk@xxxxxxxxx>
---
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.
---
v4: Re-base (over dropped cpu_has_{popcnt,bmi1,bmi2}).
v2: Also cover XSAVE.

--- a/xen/arch/x86/include/asm/cpufeature.h
+++ b/xen/arch/x86/include/asm/cpufeature.h
@@ -132,11 +132,17 @@ static inline bool boot_cpu_has(unsigned
 #define cpu_has_monitor         boot_cpu_has(X86_FEATURE_MONITOR)
 #define cpu_has_vmx             boot_cpu_has(X86_FEATURE_VMX)
 #define cpu_has_eist            boot_cpu_has(X86_FEATURE_EIST)
-#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_x2apic          boot_cpu_has(X86_FEATURE_X2APIC)
+#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_rdrand          boot_cpu_has(X86_FEATURE_RDRAND)
 #define cpu_has_hypervisor      boot_cpu_has(X86_FEATURE_HYPERVISOR)