Subject: Re: [PATCH v2 4/5] KVM: selftests: Add a test for KVM_RUN+rseq to detect task migration bugs

From: Mathieu Desnoyers <[email-masked]>
Date: Mon, 23 Aug 2021 11:18:54 -0400 (EDT)

----- On Aug 20, 2021, at 6:50 PM, Sean Christopherson [email-masked] wrote:

> Add a test to verify an rseq's CPU ID is updated correctly if the task is
> migrated while the kernel is handling KVM_RUN.  This is a regression test
> for a bug introduced by commit 72c3c0fe54a3 ("x86/kvm: Use generic xfer
> to guest work function"), where TIF_NOTIFY_RESUME would be cleared by KVM
> without updating rseq, leading to a stale CPU ID and other badness.
> 

[...]

+#define RSEQ_SIG 0xdeadbeef

Is there any reason for defining a custom signature rather than including
tools/testing/selftests/rseq/rseq.h ? This should take care of including
the proper architecture header which will define the appropriate signature.

Arguably you don't define rseq critical sections in this test per se, but
I'm wondering why the custom signature here.

[...]

> +
> +static void *migration_worker(void *ign)
> +{
> +	cpu_set_t allowed_mask;
> +	int r, i, nr_cpus, cpu;
> +
> +	CPU_ZERO(&allowed_mask);
> +
> +	nr_cpus = CPU_COUNT(&possible_mask);
> +
> +	for (i = 0; i < 20000; i++) {
> +		cpu = i % nr_cpus;
> +		if (!CPU_ISSET(cpu, &possible_mask))
> +			continue;
> +
> +		CPU_SET(cpu, &allowed_mask);
> +
> +		/*
> +		 * Bump the sequence count twice to allow the reader to detect
> +		 * that a migration may have occurred in between rseq and sched
> +		 * CPU ID reads.  An odd sequence count indicates a migration
> +		 * is in-progress, while a completely different count indicates
> +		 * a migration occurred since the count was last read.
> +		 */
> +		atomic_inc(&seq_cnt);

So technically this atomic_inc contains the required barriers because the selftests
implementation uses "__sync_add_and_fetch(&addr->val, 1)". But it's rather odd that
the semantic differs from the kernel implementation in terms of memory barriers: the
kernel implementation of atomic_inc guarantees no memory barriers, but this one
happens to provide full barriers pretty much by accident (selftests
futex/include/atomic.h documents no such guarantee).

If this full barrier guarantee is indeed provided by the selftests atomic.h header,
I would really like a comment stating that in the atomic.h header so the carpet is
not pulled from under our feet by a future optimization.

> +		r = sched_setaffinity(0, sizeof(allowed_mask), &allowed_mask);
> +		TEST_ASSERT(!r, "sched_setaffinity failed, errno = %d (%s)",
> +			    errno, strerror(errno));
> +		atomic_inc(&seq_cnt);
> +
> +		CPU_CLR(cpu, &allowed_mask);
> +
> +		/*
> +		 * Let the read-side get back into KVM_RUN to improve the odds
> +		 * of task migration coinciding with KVM's run loop.

This comment should be about increasing the odds of letting the seqlock read-side
complete. Otherwise, the delay between the two back-to-back atomic_inc is so small
that the seqlock read-side may never have time to complete the reading the rseq
cpu id and the sched_getcpu() call, and can retry forever.

I'm wondering if 1 microsecond is sufficient on other architectures as well. One
alternative way to make this depend less on the architecture's implementation of
sched_getcpu (whether it's a vDSO, or goes through a syscall) would be to read
the rseq cpu id and call sched_getcpu a few times (e.g. 3 times) in the migration
thread rather than use usleep, and throw away the value read. This would ensure
the delay is appropriate on all architectures.

Thanks!

Mathieu

> +		 */
> +		usleep(1);
> +	}
> +	done = true;
> +	return NULL;
> +}
> +
> +int main(int argc, char *argv[])
> +{
> +	struct kvm_vm *vm;
> +	u32 cpu, rseq_cpu;
> +	int r, snapshot;
> +
> +	/* Tell stdout not to buffer its content */
> +	setbuf(stdout, NULL);
> +
> +	r = sched_getaffinity(0, sizeof(possible_mask), &possible_mask);
> +	TEST_ASSERT(!r, "sched_getaffinity failed, errno = %d (%s)", errno,
> +		    strerror(errno));
> +
> +	if (CPU_COUNT(&possible_mask) < 2) {
> +		print_skip("Only one CPU, task migration not possible\n");
> +		exit(KSFT_SKIP);
> +	}
> +
> +	sys_rseq(0);
> +
> +	/*
> +	 * Create and run a dummy VM that immediately exits to userspace via
> +	 * GUEST_SYNC, while concurrently migrating the process by setting its
> +	 * CPU affinity.
> +	 */
> +	vm = vm_create_default(VCPU_ID, 0, guest_code);
> +
> +	pthread_create(&migration_thread, NULL, migration_worker, 0);
> +
> +	while (!done) {
> +		vcpu_run(vm, VCPU_ID);
> +		TEST_ASSERT(get_ucall(vm, VCPU_ID, NULL) == UCALL_SYNC,
> +			    "Guest failed?");
> +
> +		/*
> +		 * Verify rseq's CPU matches sched's CPU.  Ensure migration
> +		 * doesn't occur between sched_getcpu() and reading the rseq
> +		 * cpu_id by rereading both if the sequence count changes, or
> +		 * if the count is odd (migration in-progress).
> +		 */
> +		do {
> +			/*
> +			 * Drop bit 0 to force a mismatch if the count is odd,
> +			 * i.e. if a migration is in-progress.
> +			 */
> +			snapshot = atomic_read(&seq_cnt) & ~1;
> +			smp_rmb();
> +			cpu = sched_getcpu();
> +			rseq_cpu = READ_ONCE(__rseq.cpu_id);
> +			smp_rmb();
> +		} while (snapshot != atomic_read(&seq_cnt));
> +
> +		TEST_ASSERT(rseq_cpu == cpu,
> +			    "rseq CPU = %d, sched CPU = %d\n", rseq_cpu, cpu);
> +	}
> +
> +	pthread_join(migration_thread, NULL);
> +
> +	kvm_vm_free(vm);
> +
> +	sys_rseq(RSEQ_FLAG_UNREGISTER);
> +
> +	return 0;
> +}
> --
> 2.33.0.rc2.250.ged5fa647cd-goog

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com