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

[Alexey Dobriyan <adobriyan@gmail.com>]

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@gmail.com>
---

 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;
 	}