KVM: selftests: Randomly force emulation on x86 writes from guest code

Override vcpu_arch_put_guest() to randomly force emulation on supported
accesses.  Force emulation of LOCK CMPXCHG as well as a regular MOV to
stress KVM's emulation of atomic accesses, which has a unique path in
KVM's emulator.

Arbitrarily give all the decisions 50/50 odds; absent much, much more
sophisticated infrastructure for generating random numbers, it's highly
unlikely that doing more than a coin flip with affect selftests' ability
to find KVM bugs.

This is effectively a regression test for commit 910c57dfa4d1 ("KVM: x86:
Mark target gfn of emulated atomic instruction as dirty").

Link: https://lore.kernel.org/r/20240314185459.2439072-6-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

tools/testing/selftests/kvm/include/x86_64/kvm_util_arch.h

@@ -5,6 +5,8 @@
 #include <stdbool.h>
 #include <stdint.h>
 
+#include "test_util.h"
+
 extern bool is_forced_emulation_enabled;
 
 struct kvm_vm_arch {
@@ -22,4 +24,23 @@ static inline bool __vm_arch_has_protected_memory(struct kvm_vm_arch *arch)
 #define vm_arch_has_protected_memory(vm) \
 	__vm_arch_has_protected_memory(&(vm)->arch)
 
+#define vcpu_arch_put_guest(mem, __val)							\
+do {											\
+	const typeof(mem) val = (__val);						\
+											\
+	if (!is_forced_emulation_enabled || guest_random_bool(&guest_rng)) {		\
+		(mem) = val;								\
+	} else if (guest_random_bool(&guest_rng)) {					\
+		__asm__ __volatile__(KVM_FEP "mov %1, %0"				\
+				     : "+m" (mem)					\
+				     : "r" (val) : "memory");				\
+	} else {									\
+		uint64_t __old = READ_ONCE(mem);					\
+											\
+		__asm__ __volatile__(KVM_FEP LOCK_PREFIX "cmpxchg %[new], %[ptr]"	\
+				     : [ptr] "+m" (mem), [old] "+a" (__old)		\
+				     : [new]"r" (val) : "memory", "cc");		\
+	}										\
+} while (0)
+
 #endif  // SELFTEST_KVM_UTIL_ARCH_H