svn commit: r365652 - in head/sys: sys x86/x86

[Jason A. Harmening]

Author: jah
Date: Sat Sep 12 07:04:00 2020
New Revision: 365652
URL: https://svnweb.freebsd.org/changeset/base/365652

Log:
  amd64: prevent KCSan false positives on LAPIC mapping
  
  For configurations without x2APIC support (guests, older hardware), the global
  LAPIC MMIO mapping will trigger false-positive KCSan reports as it will appear
  that multiple CPUs are concurrently reading and writing the same address.
  This isn't actually true, as the underlying physical access will be performed
  on the local CPU's APIC. Additionally, because LAPIC access can happen during
  event timer configuration, the resulting KCSan printf can produce a panic due
  to attempted recursion on event timer resources.
  
  Add a __nosanitizethread preprocessor define to prevent the compiler from
  inserting TSan hooks, and apply it to the x86 LAPIC accessors.
  
  PR:		249149
  Reported by:	gbe
  Reviewed by:	andrew, kib
  Tested by:	gbe
  Differential Revision:	https://reviews.freebsd.org/D26354

Modified:
  head/sys/sys/cdefs.h
  head/sys/x86/x86/local_apic.c

Modified: head/sys/sys/cdefs.h
==============================================================================
--- head/sys/sys/cdefs.h	Sat Sep 12 01:55:07 2020	(r365651)
+++ head/sys/sys/cdefs.h	Sat Sep 12 07:04:00 2020	(r365652)
@@ -880,8 +880,10 @@
  */
 #if __has_attribute(no_sanitize) && defined(__clang__)
 #define __nosanitizeaddress	__attribute__((no_sanitize("address")))
+#define __nosanitizethread	__attribute__((no_sanitize("thread")))
 #else
 #define __nosanitizeaddress
+#define __nosanitizethread
 #endif
 
 /* Guard variables and structure members by lock. */

Modified: head/sys/x86/x86/local_apic.c
==============================================================================
--- head/sys/x86/x86/local_apic.c	Sat Sep 12 01:55:07 2020	(r365651)
+++ head/sys/x86/x86/local_apic.c	Sat Sep 12 07:04:00 2020	(r365652)
@@ -215,7 +215,17 @@ SYSCTL_INT(_hw_apic, OID_AUTO, timer_tsc_deadline, CTL
 static void lapic_calibrate_initcount(struct lapic *la);
 static void lapic_calibrate_deadline(struct lapic *la);
 
-static uint32_t
+/*
+ * Use __nosanitizethread to exempt the LAPIC I/O accessors from KCSan
+ * instrumentation.  Otherwise, if x2APIC is not available, use of the global
+ * lapic_map will generate a KCSan false positive.  While the mapping is
+ * shared among all CPUs, the physical access will always take place on the
+ * local CPU's APIC, so there isn't in fact a race here.  Furthermore, the
+ * KCSan warning printf can cause a panic if issued during LAPIC access,
+ * due to attempted recursive use of event timer resources.
+ */
+
+static uint32_t __nosanitizethread
 lapic_read32(enum LAPIC_REGISTERS reg)
 {
 	uint32_t res;
@@ -228,7 +238,7 @@ lapic_read32(enum LAPIC_REGISTERS reg)
 	return (res);
 }
 
-static void
+static void __nosanitizethread
 lapic_write32(enum LAPIC_REGISTERS reg, uint32_t val)
 {
 
@@ -241,7 +251,7 @@ lapic_write32(enum LAPIC_REGISTERS reg, uint32_t val)
 	}
 }
 
-static void
+static void __nosanitizethread
 lapic_write32_nofence(enum LAPIC_REGISTERS reg, uint32_t val)
 {