[RFC PATCH 03/11] x86: Add x86_vendor_is() by itself before using it

[Alejandro Vallejo <alejandro.garciavallejo@xxxxxxx>]

This function is meant to replace all instances of the following
patterns in CPU policies and boot_cpu_data:

  - x->x86_vendor == X86_VENDOR_FOO
  - x->x86_vendor != X86_VENDOR_FOO
  - x->x86_vendor & (X86_VENDOR_FOO | X86_VENDOR_BAR)

The secret sauce is that all branches inside the helper resolve at
compile time, so for the all-vendors-compiled-in case the function
resolves to equivalent code as that without the helper and you get
progressively more aggressive DCE as you disable vendors. The function
folds into a constant once you remove the fallback CPU vendor setting.

While at this, move an include out of place so they sort alphabetically.

Not a functional change.

Signed-off-by: Alejandro Vallejo <alejandro.garciavallejo@xxxxxxx>
---
 xen/arch/x86/include/asm/cpuid.h | 49 +++++++++++++++++++++++++++++++-
 1 file changed, 48 insertions(+), 1 deletion(-)

diff --git a/xen/arch/x86/include/asm/cpuid.h b/xen/arch/x86/include/asm/cpuid.h
index bf1c635cdd..a4280d1b0d 100644
--- a/xen/arch/x86/include/asm/cpuid.h
+++ b/xen/arch/x86/include/asm/cpuid.h
@@ -2,10 +2,12 @@
 #define __X86_CPUID_H__
 
 #include <asm/cpufeatureset.h>
+#include <asm/x86-vendors.h>
 
-#include <xen/types.h>
+#include <xen/compiler.h>
 #include <xen/kernel.h>
 #include <xen/percpu.h>
+#include <xen/types.h>
 
 #include <public/sysctl.h>
 
@@ -56,6 +58,51 @@ void guest_cpuid(const struct vcpu *v, uint32_t leaf,
      (IS_ENABLED(CONFIG_SHANGHAI) ? X86_VENDOR_SHANGHAI : 0) | \
      (IS_ENABLED(CONFIG_HYGON)    ? X86_VENDOR_HYGON    : 0))
 
+/*
+ * When compiling Xen for a single vendor with no fallback vendor there's no
+ * need no check the candidate. `vendor` is always a compile-time constant,
+ * which means this all can fold into a constant boolean.
+ *
+ * A runtime check at the time of CPUID probing guarantees we never run on
+ * wrong hardware and another check when loading CPU policies guarantees we
+ * never run policies for a vendor in another vendor's silicon.
+ *
+ * By the same token, the same folding can happen when no vendor is compiled
+ * in and the fallback path is present.
+ */
+static always_inline bool x86_vendor_is(uint8_t candidate, uint8_t vendor)
+{
+    uint8_t filtered_vendor = vendor & X86_ENABLED_VENDORS;
+
+    if ( vendor == X86_VENDOR_UNKNOWN )
+    {
+        if ( IS_ENABLED(CONFIG_UNKNOWN_CPU) )
+            /* no-vendor optimisation */
+            return X86_ENABLED_VENDORS ? vendor == candidate : true;
+
+        /* unknown-vendor-elimination optimisation */
+        return false;
+    }
+
+    /* single-vendor optimisation */
+    if ( !IS_ENABLED(CONFIG_UNKNOWN_CPU) &&
+         (ISOLATE_LSB(X86_ENABLED_VENDORS) == X86_ENABLED_VENDORS) )
+        return filtered_vendor == X86_ENABLED_VENDORS;
+
+    /* compiled-out-vendor-elimination optimisation */
+    if ( !filtered_vendor )
+        return false;
+
+    /*
+     * When checking against a single vendor, perform an equality check, as
+     * it yields (marginally) better codegen
+     */
+    if ( ISOLATE_LSB(filtered_vendor) == filtered_vendor )
+        return filtered_vendor == candidate ;
+
+    return filtered_vendor & candidate;
+}
+
 #endif /* !__X86_CPUID_H__ */
 
 /*
-- 
2.43.0