x86: implement prctl PR_GET_TSC and PR_SET_TSC

This patch implements the PR_GET_TSC and PR_SET_TSC prctl()
commands on the x86 platform (both 32 and 64 bit.) These
commands control the ability to read the timestamp counter
from userspace (the RDTSC instruction.)

While the RDTSC instuction is a useful profiling tool,
it is also the source of some non-determinism in ring-3.
For deterministic replay applications it is useful to be
able to trap and emulate (and record the outcome of) this
instruction.

This patch uses code earlier used to disable the timestamp
counter for the SECCOMP framework. A side-effect of this
patch is that the SECCOMP environment will now also disable
the timestamp counter on x86_64 due to the addition of the
TIF_NOTSC define on this platform.

The code which enables/disables the RDTSC instruction during
context switches is in the __switch_to_xtra function, which
already handles other unusual conditions, so normal
performance should not have to suffer from this change.

Signed-off-by: Erik Bosman <ejbosman@cs.vu.nl>
Acked-by: Arjan van de Ven  <arjan@linux.intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

1 files changed, 7 insertions, 0 deletions

diff --git a/include/asm-x86/processor.h b/include/asm-x86/processor.h
index 6e26c7c717a2..eaf4548a23d2 100644
--- a/include/asm-x86/processor.h
+++ b/include/asm-x86/processor.h
@@ -918,4 +918,11 @@ extern void start_thread(struct pt_regs *regs, unsigned long new_ip,
 
 #define KSTK_EIP(task)          (task_pt_regs(task)->ip)
 
+/* Get/set a process' ability to use the timestamp counter instruction */
+#define GET_TSC_CTL(adr)        get_tsc_mode((adr))
+#define SET_TSC_CTL(val)        set_tsc_mode((val))
+
+extern int get_tsc_mode(unsigned long adr);
+extern int set_tsc_mode(unsigned int val);
+
 #endif