KVM: selftests: Ensure all vCPUs hit -EFAULT during initial RO stage

During the initial mprotect(RO) stage of mmu_stress_test, keep vCPUs
spinning until all vCPUs have hit -EFAULT, i.e. until all vCPUs have tried
to write to a read-only page.  If a vCPU manages to complete an entire
iteration of the loop without hitting a read-only page, *and* the vCPU
observes mprotect_ro_done before starting a second iteration, then the
vCPU will prematurely fall through to GUEST_SYNC(3) (on x86 and arm64) and
get out of sequence.

Replace the "do-while (!r)" loop around the associated _vcpu_run() with
a single invocation, as barring a KVM bug, the vCPU is guaranteed to hit
-EFAULT, and retrying on success is super confusion, hides KVM bugs, and
complicates this fix.  The do-while loop was semi-unintentionally added
specifically to fudge around a KVM x86 bug, and said bug is unhittable
without modifying the test to force x86 down the !(x86||arm64) path.

On x86, if forced emulation is enabled, vcpu_arch_put_guest() may trigger
emulation of the store to memory.  Due a (very, very) longstanding bug in
KVM x86's emulator, emulate writes to guest memory that fail during
__kvm_write_guest_page() unconditionally return KVM_EXIT_MMIO.  While that
is desirable in the !memslot case, it's wrong in this case as the failure
happens due to __copy_to_user() hitting a read-only page, not an emulated
MMIO region.

But as above, x86 only uses vcpu_arch_put_guest() if the __x86_64__ guards
are clobbered to force x86 down the common path, and of course the
unexpected MMIO is a KVM bug, i.e. *should* cause a test failure.

Fixes: b6c304a ("KVM: selftests: Verify KVM correctly handles mprotect(PROT_READ)")
Reported-by: Yan Zhao <yan.y.zhao@intel.com>
Closes: https://lore.kernel.org/all/20250208105318.16861-1-yan.y.zhao@intel.com
Debugged-by: Yan Zhao <yan.y.zhao@intel.com>
Reviewed-by: Yan Zhao <yan.y.zhao@intel.com>
Tested-by: Yan Zhao <yan.y.zhao@intel.com>
Link: https://lore.kernel.org/r/20250228230804.3845860-1-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

tools/testing/selftests/kvm/mmu_stress_test.c

@@ -18,6 +18,7 @@
 #include "ucall_common.h"
 
 static bool mprotect_ro_done;
+static bool all_vcpus_hit_ro_fault;
 
 static void guest_code(uint64_t start_gpa, uint64_t end_gpa, uint64_t stride)
 {
@@ -36,9 +37,9 @@ static void guest_code(uint64_t start_gpa, uint64_t end_gpa, uint64_t stride)
 
 	/*
 	 * Write to the region while mprotect(PROT_READ) is underway.  Keep
-	 * looping until the memory is guaranteed to be read-only, otherwise
-	 * vCPUs may complete their writes and advance to the next stage
-	 * prematurely.
+	 * looping until the memory is guaranteed to be read-only and a fault
+	 * has occurred, otherwise vCPUs may complete their writes and advance
+	 * to the next stage prematurely.
 	 *
 	 * For architectures that support skipping the faulting instruction,
 	 * generate the store via inline assembly to ensure the exact length
@@ -56,7 +57,7 @@ static void guest_code(uint64_t start_gpa, uint64_t end_gpa, uint64_t stride)
 #else
 			vcpu_arch_put_guest(*((volatile uint64_t *)gpa), gpa);
 #endif
-	} while (!READ_ONCE(mprotect_ro_done));
+	} while (!READ_ONCE(mprotect_ro_done) || !READ_ONCE(all_vcpus_hit_ro_fault));
 
 	/*
 	 * Only architectures that write the entire range can explicitly sync,
@@ -81,6 +82,7 @@ struct vcpu_info {
 
 static int nr_vcpus;
 static atomic_t rendezvous;
+static atomic_t nr_ro_faults;
 
 static void rendezvous_with_boss(void)
 {
@@ -148,12 +150,16 @@ static void *vcpu_worker(void *data)
 	 * be stuck on the faulting instruction for other architectures.  Go to
 	 * stage 3 without a rendezvous
 	 */
-	do {
-		r = _vcpu_run(vcpu);
-	} while (!r);
+	r = _vcpu_run(vcpu);
 	TEST_ASSERT(r == -1 && errno == EFAULT,
 		    "Expected EFAULT on write to RO memory, got r = %d, errno = %d", r, errno);
 
+	atomic_inc(&nr_ro_faults);
+	if (atomic_read(&nr_ro_faults) == nr_vcpus) {
+		WRITE_ONCE(all_vcpus_hit_ro_fault, true);
+		sync_global_to_guest(vm, all_vcpus_hit_ro_fault);
+	}
+
 #if defined(__x86_64__) || defined(__aarch64__)
 	/*
 	 * Verify *all* writes from the guest hit EFAULT due to the VMA now
@@ -378,7 +384,6 @@ int main(int argc, char *argv[])
 	rendezvous_with_vcpus(&time_run2, "run 2");
 
 	mprotect(mem, slot_size, PROT_READ);
-	usleep(10);
 	mprotect_ro_done = true;
 	sync_global_to_guest(vm, mprotect_ro_done);