tools/arm: Reject configuration with incorrect nr_spis value

If the calculated value for nr_spis by the toolstack is bigger than the
value provided by the user, we silently ignore the latter. This is not
consistent with the approach we have in Xen on Arm when we try to reject
incorrect configuration. Also, the documentation for nr_spis is
incorrect as it mentions 991 as the number of max SPIs, where it should
be 960 i.e. (1020 - 32) rounded down to the nearest multiple of 32.

Signed-off-by: Michal Orzel <michal.orzel@amd.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>

docs/man/xl.cfg.5.pod.in

@@ -3083,14 +3083,11 @@ interval of colors (such as "0-4").
 =item B<nr_spis="NR_SPIS">
 
 An optional integer parameter specifying the number of SPIs (Shared
-Peripheral Interrupts) to allocate for the domain. Max is 991 SPIs. If
-the value specified by the `nr_spis` parameter is smaller than the
-number of SPIs calculated by the toolstack based on the devices
-allocated for the domain, or the `nr_spis` parameter is not specified,
-the value calculated by the toolstack will be used for the domain.
-Otherwise, the value specified by the `nr_spis` parameter will be used.
-The number of SPIs should match the highest interrupt ID that will be
-assigned to the domain.
+Peripheral Interrupts) to allocate for the domain. Max is 960 SPIs. If
+the `nr_spis` parameter is not specified, the value calculated by the toolstack
+will be used for the domain. Otherwise, the value specified by the `nr_spis`
+parameter will be used. The number of SPIs should match the highest interrupt
+ID that will be assigned to the domain.
 
 =back
 

tools/libs/light/libxl_arm.c

@@ -181,6 +181,12 @@ int libxl__arch_domain_prepare_config(libxl__gc *gc,
 
     LOG(DEBUG, "Configure the domain");
 
+    if (nr_spis > d_config->b_info.arch_arm.nr_spis) {
+        LOG(ERROR, "Provided nr_spis value is too small (minimum required %u)\n",
+            nr_spis);
+        return ERROR_FAIL;
+    }
+
     config->arch.nr_spis = max(nr_spis, d_config->b_info.arch_arm.nr_spis);
     LOG(DEBUG, " - Allocate %u SPIs", config->arch.nr_spis);