KVM: selftests: Add guest udelay() utility for x86

Add udelay() for x86 tests to allow busy waiting in the guest for a
specific duration, and to match ARM and RISC-V's udelay() in the hopes
of eventually making udelay() available on all architectures.

Get the guest's TSC frequency using KVM_GET_TSC_KHZ and expose it to all
VMs via a new global, guest_tsc_khz.  Assert that KVM_GET_TSC_KHZ returns
a valid frequency, instead of simply skipping tests, which would require
detecting which tests actually need/want udelay().  KVM hasn't returned an
error for KVM_GET_TSC_KHZ since commit cc578287e3 ("KVM: Infrastructure
for software and hardware based TSC rate scaling"), which predates KVM
selftests by 6+ years (KVM_GET_TSC_KHZ itself predates KVM selftest by 7+
years).

Note, if the GUEST_ASSERT() in udelay() somehow fires and the test doesn't
check for guest asserts, then the test will fail with a very cryptic
message.  But fixing that, e.g. by automatically handling guest asserts,
is a much larger task, and practically speaking the odds of a test afoul
of this wart are infinitesimally small.

Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Link: https://lore.kernel.org/r/5aa86285d1c1d7fe1960e3fe490f4b22273977e6.1718214999.git.reinette.chatre@intel.com
Co-developed-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>

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

@@ -23,6 +23,7 @@
 
 extern bool host_cpu_is_intel;
 extern bool host_cpu_is_amd;
+extern uint64_t guest_tsc_khz;
 
 /* Forced emulation prefix, used to invoke the emulator unconditionally. */
 #define KVM_FEP "ud2; .byte 'k', 'v', 'm';"
@@ -815,6 +816,23 @@ static inline void cpu_relax(void)
 	asm volatile("rep; nop" ::: "memory");
 }
 
+static inline void udelay(unsigned long usec)
+{
+	uint64_t start, now, cycles;
+
+	GUEST_ASSERT(guest_tsc_khz);
+	cycles = guest_tsc_khz / 1000 * usec;
+
+	/*
+	 * Deliberately don't PAUSE, a.k.a. cpu_relax(), so that the delay is
+	 * as accurate as possible, e.g. doesn't trigger PAUSE-Loop VM-Exits.
+	 */
+	start = rdtsc();
+	do {
+		now = rdtsc();
+	} while (now - start < cycles);
+}
+
 #define ud2()			\
 	__asm__ __volatile__(	\
 		"ud2\n"	\