Re: [RFC PATCH v1 4/4] KVM: selftests: Add bus lock exit test

[Sean Christopherson <seanjc@google.com>]

On Mon, Aug 26, 2024, Manali Shukla wrote:
> >> +struct buslock_test {
> >> +	unsigned char pad[126];
> >> +	atomic_long_t val;
> >> +} __packed;
> >> +
> >> +struct buslock_test test __cacheline_aligned;
> >> +
> >> +static __always_inline void buslock_atomic_add(int i, atomic_long_t *v)
> >> +{
> >> +	asm volatile(LOCK_PREFIX "addl %1,%0"
> >> +		     : "+m" (v->counter)
> >> +		     : "ir" (i) : "memory");
> >> +}
> >> +
> >> +static void buslock_add(void)
> >> +{
> >> +	/*
> >> +	 * Increment a cache unaligned variable atomically.
> >> +	 * This should generate a bus lock exit.
> > 
> > So... this test doesn't actually verify that a bus lock exit occurs.  The userspace
> > side will eat an exit if one occurs, but there's literally not a single TEST_ASSERT()
> > in here.
> 
> Agreed, How about doing following?
> 
> +       for (;;) {
> +               struct ucall uc;
> +
> +               vcpu_run(vcpu);
> +
> +               if (run->exit_reason == KVM_EXIT_IO) {
> +                       switch (get_ucall(vcpu, &uc)) {
> +                       case UCALL_ABORT:
> +                               REPORT_GUEST_ASSERT(uc);
> +                               /* NOT REACHED */
> +                       case UCALL_SYNC:
> +                               break;
> +                       case UCALL_DONE:
> +                               goto done;
> +                       default:
> +                               TEST_FAIL("Unknown ucall 0x%lx.", uc.cmd);
> +                       }
> +               }
> +
> +               TEST_ASSERT_KVM_EXIT_REASON(vcpu, KVM_EXIT_X86_BUS_LOCK);

I doubt this works, the UCALL_SYNC above will fallthrough to this assert.  I
assume run->exit_reason needs a continue for UCALL_SYNC.

> +               TEST_ASSERT_EQ(run->flags, KVM_RUN_X86_BUS_LOCK);
> +               run->flags &= ~KVM_RUN_X86_BUS_LOCK;

No need, KVM should clear the flag if the exit isn't due to a bus lock.

> +               run->exit_reason = 0;

Again, no need, KVM should take care of resetting exit_reason.

> +       }
> 
> - Manali
> 
>