update compat

[lttv.git] / doc / developer / tsc.txt

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AMD
http://lkml.org/lkml/2005/11/4/173
http://www.amd.com/us-en/assets/content_type/white_papers_and_tech_docs/26094.PDF
http://developer.amd.com/article_print.jsp?id=92 (RH)

[7] AMD's 7th  generation  processors return  a CPUID  base
    family value of '7'. These include AMD Athlon, AthlonXP,
    AthlonMP, and Duron.

[8] AMD's  8th generation  processors  return an  effective
    CPUID family of '0x0F'. These include AMD Opteron, 
    Athlon64, and Turion.


before 7th gen : ok
7th gen:
  P-state (performance state) change
    UP : warn about time inaccuracy
    SMP
      sol : disable powernow
      Use monotonic pseudo-TSC
  STPCLK-Throttling (temperature) : only done on UP, ok
    UP : warn about time inaccuracy
8th gen :
  P-state change
    UP : inaccuracy
    dual-core : locked-step ; inaccuracy
    SMP : may drift
      sol : disable powernow
    Use monotonic pseudo-TSC
  SMP, dual core : C1-clock ramping (halt) (power state : C-state)
    sol : idle=poll or disable C1-ramping
    Use monotonic pseudo-TSC
  STPCLK-Throttling (temperature) :
    single processor dual-core ok ; inaccuracy
    SMP : NOT ok (rare)
      Use monotonic pseudo-TSC


Until TSC  becomes invariant, AMD  recommends that operating
system  developers  avoid TSC  as  a  fast  timer source  on
affected systems. (AMD  recommends that the operating system
should  favor these  time sources  in a  prioritized manner:
HPET first,  then ACPI PM  Timer, then PIT.)   The following
pseudo-code shows one way of determining when to use TSC:

 use_AMD_TSC() { // returns TRUE if ok to use TSC
   if (CPUID.base_family < 0xf) {
             // TSC drift doesn't exist on 7th Gen or less
             // However, OS still needs to consider effects
             // of P-state changes on TSC
             return TRUE;
   } else if (CPUID.AdvPowerMgmtInfo.TscInvariant) {
             // Invariant TSC on 8th Gen or newer, use it
             // (assume all cores have invariant TSC)
             return TRUE;
   } else if ((number_processors == 1)&&(number_cores == 1)){
             // OK to use TSC on uni-processor-uni-core
             // However, OS still needs to consider effects
             // of P-state changes on TSC
             return TRUE;
   } else if ( (number_processors == 1) && 
               (CPUID.effective_family == 0x0f) &&
               !C1_ramp_8gen                       ){
             // Use TSC on 8th Gen uni-proc with C1_ramp off 
             // However, OS still needs to consider effects
             // of P-state changes on TSC
             return TRUE;
   } else {
             return FALSE;
   }
 }
 C1_ramp_8gen() {
    // Check if C1-Clock ramping enabled in  PMM7.CpuLowPwrEnh
    // On 8th-Generation cores only. Assume BIOS has setup
    // all Northbridges equivalently.
    return (1 & read_pci_byte(bus=0,dev=0x18,fcn=3,offset=0x87));
 }


When  an operating  system can  not avoid  using TSC  in the
short-term,  the  operating   system  will  need  to  either
re-synchronize the TSC of  the halted core when exiting halt
or disable C1-clock  ramping.  The pseudo-code for disabling
C1-clock ramping follows:

 if ( !use_AMD_TSC() && 
      (CPUID.effective_family == 0x0f) &&
      C1_ramp_8gen()                       ){
    for (i=0; i < number_processors; ++i){
       // Do for all NorthBridges in platform
       tmp = read_pci_byte(bus=0,dev=0x18+i,fcn=3,offset=0x87);
       tmp &= 0xFC;    // clears pmm7[1:0]
       write_pci_byte(bus=0,dev=0x18+i,fcn=3,offset=0x87,tmp)
      }
 }


Future TSC Directions and Solutions
===================================
Future AMD processors will provide a TSC that is P-state and
C-State invariant and unaffected by STPCLK-throttling.  This
will make  the TSC immune  to drift.  Because using  the TSC
for  fast  timer APIs  is  a  desirable  feature that  helps
performance,  AMD  has  defined  a CPUID  feature  bit  that
software   can   test   to   determine   if   the   TSC   is
invariant. Issuing a CPUID instruction with an %eax register
value of  0x8000_0007, on a  processor whose base  family is
0xF, returns "Advanced  Power Management Information" in the
%eax, %ebx, %ecx,  and %edx registers.  Bit 8  of the return
%edx is  the "TscInvariant" feature  flag which is  set when
TSC is P-state, C-state, and STPCLK-throttling invariant; it
is clear otherwise.

The  rate of the  invariant TSC  is implementation-dependent
and  will likely  *not* be  the frequency  of  the processor
core; however,  its period should be short  enough such that
it is  not possible for two  back-to-back rdtsc instructions
to  return the  same  value.  Software  which  is trying  to
measure  actual  processor  frequency  or  cycle-performance
should  use Performance  Event 76h,  CPU Clocks  not Halted,
rather than the TSC to count CPU cycles.



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Intel

Pentium M
  family [06H], models [09H, 0DH]
  UP
    warn about time inaccuracy
  SMP
    SOL : disable speedstep
Pentium 4 processors, Intel Xeon
  family [0FH], models [00H, 01H, or 02H]
  UP
    warn about time inaccuracy
  SMP
    SOL : disable speedstep

Other : ok

http://download.intel.com/design/Xeon/specupdt/30675712.pdf
http://forum.rightmark.org/post.cgi?id=print:6:269&user=%20Dmitri%20Besedin&color=1
http://bochs.sourceforge.net/techspec/24161821.pdf cpuid

cpuz
AFAIK this problem only concerns Prescott CPUs, but I bet future production will
also use the same rule.

Well, from what Intel states in one of their docs (Intel(R) Pentium(R) M
Processor on 90 nm Process with 2-MB L2 Cache, Specification Update, Document
No. 302209-010 (http://www.intel.com/design/mobile/specupdt/302209.htm) ), your
supposition is true:


Article V. For Pentium M processors (family [06H], models [09H, 0DH]); for
Pentium 4 processors, Intel Xeon processors (family [0FH], models [00H, 01H, or
02H]); and for P6 family processors: the timestamp counter increments with every
internal processor clock cycle. The internal processor clock cycle is determined
by the current core-clock to bus-clock ratio. Intel(R) SpeedStep(R) technology
transitions may also impact the processor clock.

Article VI. For Pentium 4 processors, Intel Xeon processors (family [0FH],
models [03H and higher]): the time-stamp counter increments at a constant rate.
That rate may be set by the maximum core-clock to bus-clock ratio of the
processor or may be set by the frequency at which the processor is booted. The
specific processor configuration determines the behavior. Constant TSC behavior
ensures that the duration of each clock tick is uniform and supports the use of
the TSC as a wall clock timer even if the processor core changes frequency. This
is the architectural behavior moving forward.


It's a pity they call this sucking TSC behavior an "architectural behavior
moving forward"



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HPET

http://www.intel.com/hardwaredesign/hpetspec_1.pdf