[PATCH 2/3] x86/cpu: fix unbootable VMs by inlining memcmp in hypervisor_cpuid_base

[Alexey Dobriyan <adobriyan@xxxxxxxxx>]

If this memcmp() is not inlined then PVH early boot code can call
into KASAN-instrumented memcmp() which results in unbootable VMs:

        pvh_start_xen
        xen_prepare_pvh
        xen_cpuid_base
        hypervisor_cpuid_base
        memcmp

Ubuntu's gcc version 11.4.0 (Ubuntu 11.4.0-1ubuntu1~22.04) inlines
memcmp with patch and the bug is partially fixed.

Leave FIXME just in case someone cares enough to compare 3 pairs of
integers like 3 pairs of integers.

Signed-off-by: Alexey Dobriyan <adobriyan@xxxxxxxxx>
---

 arch/x86/include/asm/cpuid.h | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/cpuid.h b/arch/x86/include/asm/cpuid.h
index 6b122a31da06..3eca7824430e 100644
--- a/arch/x86/include/asm/cpuid.h
+++ b/arch/x86/include/asm/cpuid.h
@@ -196,7 +196,20 @@ static inline uint32_t hypervisor_cpuid_base(const char 
*sig, uint32_t leaves)
        for_each_possible_hypervisor_cpuid_base(base) {
                cpuid(base, &eax, &signature[0], &signature[1], &signature[2]);
 
-               if (!memcmp(sig, signature, 12) &&
+               /*
+                * FIXME rewrite cpuid comparators to accept uint32_t[3].
+                *
+                * This memcmp()
+                * a) is called from PVH early boot code
+                *    before instrumentation is set up,
+                * b) may be compiled to "call memcmp" (not inlined),
+                * c) memcmp() itself may be instrumented.
+                *
+                * Any combination of 2 is fine, but all 3 aren't.
+                *
+                * Force inline this function call.
+                */
+               if (!__builtin_memcmp(sig, signature, 12) &&
                    (leaves == 0 || ((eax - base) >= leaves)))
                        return base;
        }