RE: [PATCH] x86: Export tsc related information in sysfs

[Dan Magenheimer <dan.magenheimer@oracle.com>]

> From: H. Peter Anvin [mailto:hpa@zytor.com]
> Sent: Tuesday, May 18, 2010 11:04 AM
> To: Peter Zijlstra
> Cc: Andi Kleen; Arjan van de Ven; Dan Magenheimer; Thomas Gleixner;
> Venkatesh Pallipadi; Ingo Molnar; Chris Mason; linux-
> kernel@vger.kernel.org
> Subject: Re: [PATCH] x86: Export tsc related information in sysfs
> 
> On 05/18/2010 09:52 AM, Peter Zijlstra wrote:
> > On Tue, 2010-05-18 at 09:40 -0700, H. Peter Anvin wrote:
> >>
> >> The problem is that you throw the baby (vsyscall) out with the
> bathwater
> >> (user rdtsc).
> >
> > Well, we could only flip the CR4 bit when we mark the TSC unsuitable
> for
> > gtod. That should be plenty good to tag all userspace trying to use
> it,
> > since more than half my machines don't use TSC for clocksource.
> 
> This might be an option, although it would have to be an *option*.
> There are restricted uses of the TSC in userspace which are still
> useful
> (mainly involving performance analysis and/or CPU-locked processes).

(Though I expect tglx/arjan/andi/mingo to disagree with this proposal
for similar reasons as the original one that started this thread...)

Proposal:

/sys/devices/system/tsc/native (writable by root):

0 = (default) Kernel dynamically controls TSC emulation.
  When the kernel deems TSC usable as a clocksource, rdtsc
  will be executed directly by the CPU.  When the kernel deems
  TSC unsafe to use, rdtsc will be trapped and emulated.
1 = TSC emulation is never enabled.  Programs using rdtsc
  directly are subject to the many known and sometimes rare
  and subtle vagaries of TSC.
2 = TSC emulation is always enabled (for debug only)
3 = Processes using TSC will be treated as if they executed
  an illegal instruction.  [? Can the kernel recognize
  use of rdtsc in a vsyscall and emulate so that,
  even though vsyscall is slower, all other rdtsc
  in userspace are illegal?] [? Can/should this be
  enforced only on non-root processes?]

/sys/devices/system/tsc/system_count (writable by root):

Contains a count of all TSC emulations, system-wide.
Writable to allow reset to zero.

/sys/devices/system/tsc/pid_counters (writable by root):

0 = (default) TSC counts are system-wide only
1 = TSC counted per pid (at performance penalty)
    counters in /proc/PID/tsc_count

/proc/PID/tsc_count (readonly):

If /sys/devices/system/tsc_pid_counters is 1,
contains the count of rdtsc instructions emulated
for this PID.

(Note: except for the actual instruction emulation
which will be faithful, rdtscp will be treated and
counted as a rdtsc.)