x86/hyperv: Avoid erroneously sending IPI to 'self'

__send_ipi_mask_ex() uses an optimization: when the target CPU mask is
equal to 'cpu_present_mask' it uses 'HV_GENERIC_SET_ALL' format to avoid
converting the specified cpumask to VP_SET. This case was overlooked when
'exclude_self' parameter was added. As the result, a spurious IPI to
'self' can be send.
Reported-by: Thomas Gleixner <tglx@linutronix.de>
Fixes: dfb5c1e1 ("x86/hyperv: remove on-stack cpumask from hv_send_ipi_mask_allbutself")
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Reviewed-by: Michael Kelley <mikelley@microsoft.com>
Link: https://lore.kernel.org/r/20211006125016.941616-1-vkuznets@redhat.comSigned-off-by: Wei Liu <wei.liu@kernel.org>

arch/x86/hyperv/hv_apic.c

@@ -122,17 +122,27 @@ static bool __send_ipi_mask_ex(const struct cpumask *mask, int vector,
 	ipi_arg->reserved = 0;
 	ipi_arg->vp_set.valid_bank_mask = 0;
 
-	if (!cpumask_equal(mask, cpu_present_mask)) {
+	/*
+	 * Use HV_GENERIC_SET_ALL and avoid converting cpumask to VP_SET
+	 * when the IPI is sent to all currently present CPUs.
+	 */
+	if (!cpumask_equal(mask, cpu_present_mask) || exclude_self) {
 		ipi_arg->vp_set.format = HV_GENERIC_SET_SPARSE_4K;
 		if (exclude_self)
 			nr_bank = cpumask_to_vpset_noself(&(ipi_arg->vp_set), mask);
 		else
 			nr_bank = cpumask_to_vpset(&(ipi_arg->vp_set), mask);
-	}
-	if (nr_bank < 0)
+
+		/*
+		 * 'nr_bank <= 0' means some CPUs in cpumask can't be
+		 * represented in VP_SET. Return an error and fall back to
+		 * native (architectural) method of sending IPIs.
+		 */
+		if (nr_bank <= 0)
 			goto ipi_mask_ex_done;
-	if (!nr_bank)
+	} else {
 		ipi_arg->vp_set.format = HV_GENERIC_SET_ALL;
+	}
 
 	status = hv_do_rep_hypercall(HVCALL_SEND_IPI_EX, 0, nr_bank,
 			      ipi_arg, NULL);