[RFC PATCH 04/11] x86: Refactor early vendor lookup code to use x86_vendor_is()

[Alejandro Vallejo <alejandro.garciavallejo@xxxxxxx>]

Move the "unknown" vendor ahead of all others and have it NOT rely
on x86_vendor_is(), as that would yield incorrect values for the
single-vendor+no-fallback case when running in incorrect hardware
(because x86_vendor_is() becomes a folded constant of the single
compiled-in vendor).

This is one of the two places where x86_vendor_is() cannot be used,
along with the compatibility check on loaded guest CPU policies.

Signed-off-by: Alejandro Vallejo <alejandro.garciavallejo@xxxxxxx>
---
 xen/arch/x86/cpu/common.c | 31 +++++++++++++++++++++++--------
 1 file changed, 23 insertions(+), 8 deletions(-)

diff --git a/xen/arch/x86/cpu/common.c b/xen/arch/x86/cpu/common.c
index 393c30227f..c0c3606dd2 100644
--- a/xen/arch/x86/cpu/common.c
+++ b/xen/arch/x86/cpu/common.c
@@ -342,23 +342,38 @@ void __init early_cpu_init(bool verbose)
 
        c->x86_vendor = x86_cpuid_lookup_vendor(ebx, ecx, edx) &
                        X86_ENABLED_VENDORS;
-       switch (c->x86_vendor) {
-       case X86_VENDOR_INTEL:    intel_unlock_cpuid_leaves(c);
-                                 actual_cpu = intel_cpu_dev;    break;
-       case X86_VENDOR_AMD:      actual_cpu = amd_cpu_dev;      break;
-       case X86_VENDOR_CENTAUR:  actual_cpu = centaur_cpu_dev;  break;
-       case X86_VENDOR_SHANGHAI: actual_cpu = shanghai_cpu_dev; break;
-       case X86_VENDOR_HYGON:    actual_cpu = hygon_cpu_dev;    break;
-       default:
+
+       /*
+        * We can't rely on x86_vendor_is() here due to the single-vendor
+        * optimisation. It makes x86_vendor_is(x, y) rely on the constant `y`
+        * matching the single vendor Xen was compiled for and ignore the
+        * runtime variable `x`. In order to preserve sanity we must assert here
+        * that we never boot such a build in a CPU from another vendor, or
+        * major chaos would ensue.
+        */
+       if (c->x86_vendor == X86_VENDOR_UNKNOWN)
+       {
                if (verbose || !IS_ENABLED(CONFIG_UNKNOWN_CPU))
                        printk(XENLOG_ERR
                               "Unrecognised or unsupported CPU vendor 
'%.12s'\n",
                               c->x86_vendor_id);
+
                if (!IS_ENABLED(CONFIG_UNKNOWN_CPU))
                        panic("Cannot run in unknown/compiled-out CPU 
vendor.\n");
 
                actual_cpu = default_cpu;
        }
+       else if (x86_vendor_is(c->x86_vendor, X86_VENDOR_INTEL)) {
+               intel_unlock_cpuid_leaves(c);
+               actual_cpu = intel_cpu_dev;
+       } else if (x86_vendor_is(c->x86_vendor, X86_VENDOR_AMD))
+               actual_cpu = amd_cpu_dev;
+       else if (x86_vendor_is(c->x86_vendor, X86_VENDOR_CENTAUR))
+               actual_cpu = centaur_cpu_dev;
+       else if (x86_vendor_is(c->x86_vendor, X86_VENDOR_SHANGHAI))
+               actual_cpu = shanghai_cpu_dev;
+       else if (x86_vendor_is(c->x86_vendor, X86_VENDOR_HYGON))
+               actual_cpu = hygon_cpu_dev;
 
        cpuid(0x00000001, &eax, &ebx, &ecx, &edx);
        c->x86 = get_cpu_family(eax, &c->x86_model, &c->x86_mask);
-- 
2.43.0