Re: Calculate the frequency of the tsc timecounter

On Mon, Jul 31, 2017 at 3:58 PM, Mike Belopuhov <mike@belopuhov.com> wrote:
> On Mon, Jul 31, 2017 at 09:48 +0800, Adam Steen wrote:
>> Ted Unangst wrote:
>> > we don't currently export this info, but we could add some sysctls. there's
>> > some cpufeatures stuff there, but generally stuff isn't exported until
>> > somebody finds a use for it... it shouldn't be too hard to add something to
>> > amd64/machdep.c sysctl if you're interested.
>>
>> I am interested, as i need the info, i will look into it and hopefully
>> come back with a patch.
>
> This is a bad idea because TSC as the time source is only usable
> by OpenBSD on Skylake and Kaby Lake CPUs since they encode the TSC
> frequency in the CPUID. All older CPUs have their TSCs measured
> against the PIT. Currently the measurement done by the kernel isn't
> very precise and if TSC is selected as a timecounter, the machine
> would be gaining time on a pace that cannot be corrected by our NTP
> daemon. (IIRC, about an hour a day on my Haswell running with NTP).
>
> To be able to use TSC as a timecounter source on OpenBSD or Solo5
> you'd have to improve the in-kernel measurement of the TSC frequency
> first. I've tried to perform 10 measurements and take an average and
> it does improve accuracy, however I believe we need to poach another
> bit from Linux and re-calibrate TSC via HPET:
>
> http://elixir.free-electrons.com/linux/v4.12.4/source/arch/x86/kernel/tsc.c#L409
>
> I think this is the most sane thing we can do. Here's a complete
> procedure that Linux kernel undertakes:
>
> http://elixir.free-electrons.com/linux/v4.12.4/source/arch/x86/kernel/tsc.c#L751
>
> Regards,
> Mike

Hi Mike and Ted

I understand using the tsc as a timecounter on non Skylake and
Kabylake processors is inaccurate, but this i my first real foray into
kernel programming, so wanted to started of slow. below is a diff to
expose if the tsc is invariant and the tsc frequency via sysctl
machdep. I would like to get this commited first and then move on to
improving the in-kernel measurement of the tsc frequency as Mike
describes above.

Sorry its taken a while to get back to you I have been working with
Mike Larkin on vmm and my port of Solo5/ukvm.

Cheers
Adam

comments?

Index: sys/arch/amd64/amd64/identcpu.c
===================================================================
RCS file: /cvs/src/sys/arch/amd64/amd64/identcpu.c,v
retrieving revision 1.87
diff -u -p -u -p -r1.87 identcpu.c
--- sys/arch/amd64/amd64/identcpu.c 20 Jun 2017 05:34:41 -0000 1.87
+++ sys/arch/amd64/amd64/identcpu.c 2 Aug 2017 23:45:54 -0000
@@ -63,6 +63,8 @@ struct timecounter tsc_timecounter = {
tsc_get_timecount, NULL, ~0u, 0, "tsc", -1000, NULL
};

+u_int64_t amd64_tsc_freq = 0;
+int amd64_has_invariant_tsc;
int amd64_has_xcrypt;
#ifdef CRYPTO
int amd64_has_pclmul;
@@ -566,9 +568,12 @@ identifycpu(struct cpu_info *ci)
/* Check if it's an invariant TSC */
if (cpu_apmi_edx & CPUIDEDX_ITSC)
ci->ci_flags |= CPUF_INVAR_TSC;
+
+ amd64_has_invariant_tsc = (ci->ci_flags & CPUF_INVAR_TSC) != 0;
}

ci->ci_tsc_freq = cpu_tsc_freq(ci);
+ amd64_tsc_freq = ci->ci_tsc_freq;

amd_cpu_cacheinfo(ci);

Index: sys/arch/amd64/amd64/machdep.c
===================================================================
RCS file: /cvs/src/sys/arch/amd64/amd64/machdep.c,v
retrieving revision 1.231
diff -u -p -u -p -r1.231 machdep.c
--- sys/arch/amd64/amd64/machdep.c 12 Jul 2017 06:26:32 -0000 1.231
+++ sys/arch/amd64/amd64/machdep.c 2 Aug 2017 23:45:54 -0000
@@ -425,7 +425,9 @@ int
cpu_sysctl(int *name, u_int namelen, void *oldp, size_t *oldlenp, void *newp,
size_t newlen, struct proc *p)
{
+ extern u_int64_t amd64_tsc_freq;
extern int amd64_has_xcrypt;
+ extern int amd64_has_invariant_tsc;
dev_t consdev;
dev_t dev;
int val, error;
@@ -496,6 +498,10 @@ cpu_sysctl(int *name, u_int namelen, voi
pckbc_release_console();
return (error);