tile: nohz: warn if nohz_full uses hypervisor shared cores

The "hypervisor shared" cores are ones that the Tilera hypervisor
uses to receive interrupts to manage hypervisor-owned devices.
It's a bad idea to try to use those cores with nohz_full, since
they will get interrupted unpredictably -- and invisibly to Linux
tracing tools, since the interrupts are delivered at a higher
privilege level to the Tilera hypervisor.

Generate a clear warning at boot up that this doesn't end well
for the nohz_full cores in question.

Signed-off-by: Chris Metcalf <cmetcalf@ezchip.com>

arch/tile/include/hv/hypervisor.h

@@ -961,7 +961,11 @@ typedef enum {
   HV_INQ_TILES_HFH_CACHE       = 2,
 
   /** The set of tiles that can be legally used as a LOTAR for a PTE. */
-  HV_INQ_TILES_LOTAR           = 3
+  HV_INQ_TILES_LOTAR           = 3,
+
+  /** The set of "shared" driver tiles that the hypervisor may
+   *  periodically interrupt. */
+  HV_INQ_TILES_SHARED          = 4
 } HV_InqTileSet;
 
 /** Returns specific information about various sets of tiles within the

arch/tile/kernel/setup.c

@@ -32,6 +32,7 @@
 #include <linux/hugetlb.h>
 #include <linux/start_kernel.h>
 #include <linux/screen_info.h>
+#include <linux/tick.h>
 #include <asm/setup.h>
 #include <asm/sections.h>
 #include <asm/cacheflush.h>
@@ -1390,6 +1391,28 @@ static int __init dataplane(char *str)
 
 early_param("dataplane", dataplane);
 
+#ifdef CONFIG_NO_HZ_FULL
+/* Warn if hypervisor shared cpus are marked as nohz_full. */
+static int __init check_nohz_full_cpus(void)
+{
+	struct cpumask shared;
+	int cpu;
+
+	if (hv_inquire_tiles(HV_INQ_TILES_SHARED,
+			     (HV_VirtAddr) shared.bits, sizeof(shared)) < 0) {
+		pr_warn("WARNING: No support for inquiring hv shared tiles\n");
+		return 0;
+	}
+	for_each_cpu(cpu, &shared) {
+		if (tick_nohz_full_cpu(cpu))
+			pr_warn("WARNING: nohz_full cpu %d receives hypervisor interrupts!\n",
+			       cpu);
+	}
+	return 0;
+}
+arch_initcall(check_nohz_full_cpus);
+#endif
+
 #ifdef CONFIG_CMDLINE_BOOL
 static char __initdata builtin_cmdline[COMMAND_LINE_SIZE] = CONFIG_CMDLINE;
 #endif