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Index: sys/amd64/amd64/initcpu.c
===================================================================
--- sys/amd64/amd64/initcpu.c
+++ sys/amd64/amd64/initcpu.c
@@ -42,6 +42,7 @@
#include <machine/cputypes.h>
#include <machine/md_var.h>
+#include <machine/psl.h>
#include <machine/specialreg.h>
#include <vm/vm.h>
@@ -218,11 +219,24 @@
}
/*
- * The value for the TSC_AUX MSR and rdtscp/rdpid.
+ * The value for the TSC_AUX MSR and rdtscp/rdpid on the invoking CPU.
+ *
+ * Caller should prevent CPU migration from before the call until after
+ * consuming the call's return value.
*/
u_int
cpu_auxmsr(void)
{
+ /*
+ * Although disabling interrupts is, of course, not the only way to
+ * prevent CPU migration, since all existing callers happen to do
+ * this, make this the precedent for future callers until there is
+ * a reason to do otherwise.
+ */
+ KASSERT((read_rflags() & PSL_I) == 0,
+ ("cpu_auxmsr() returns a CPU-specific value; caller "
+ "should prevent migration until after consuming it."));
+
return (PCPU_GET(cpuid));
}
Index: sys/i386/i386/initcpu.c
===================================================================
--- sys/i386/i386/initcpu.c
+++ sys/i386/i386/initcpu.c
@@ -41,6 +41,7 @@
#include <machine/cputypes.h>
#include <machine/md_var.h>
+#include <machine/psl.h>
#include <machine/specialreg.h>
#include <vm/vm.h>
@@ -628,11 +629,24 @@
#endif
/*
- * The value for the TSC_AUX MSR and rdtscp/rdpid.
+ * The value for the TSC_AUX MSR and rdtscp/rdpid on the invoking CPU.
+ *
+ * Caller should prevent CPU migration from before the call until after
+ * consuming the call's return value.
*/
u_int
cpu_auxmsr(void)
{
+ /*
+ * Although disabling interrupts is, of course, not the only way to
+ * prevent CPU migration, since all existing callers happen to do
+ * this, make this the precedent for future callers until there is
+ * a reason to do otherwise.
+ */
+ KASSERT((read_eflags() & PSL_I) == 0,
+ ("cpu_auxmsr() returns a CPU-specific value; caller "
+ "should prevent migration until after consuming it."));
+
return (PCPU_GET(cpuid));
}

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