Re: [PATCH 5/5] x86/time: implement PVCLOCK_TSC_STABLE_BIT

[Joao Martins <joao.m.martins@oracle.com>]

On 03/18/2016 08:58 PM, Andrew Cooper wrote:
> On 17/03/16 16:12, Joao Martins wrote:
>> When using TSC as clocksource we will solely rely on TSC for updating
>> vcpu time infos (pvti). Right now, each vCPU takes the tsc_timestamp at
>> different instants meaning every EPOCH + delta. This delta is variable
>> depending on the time the CPU calibrates with CPU 0 (master), and will
>> likely be different and variable across vCPUS. This means that each VCPU
>> pvti won't account to its calibration error which could lead to time
>> going backwards, and allowing a situation where time read on VCPU B
>> immediately after A being smaller. While this doesn't happen a lot, I
>> was able to observe (for clocksource=tsc) around 50 times in an hour
>> having warps of < 100 ns.
>>
>> This patch proposes relying on host TSC synchronization and passthrough
>> of the master tsc to the guest, when running on a TSC-safe platform.  On
>> the rendezvous function we will retrieve the platform time in ns and the
>> last count read by the clocksource that was used to compute system time.
>> master will write both master_tsc_stamp and master_stime, and the other
>> vCPUS (slave) will use it to update their correspondent time infos.
>> This way we can guarantee that on a platform with a constant and
>> reliable TSC, that the time read on vcpu B right after A is bigger
>> independently of the VCPU calibration error. Since pvclock time infos
>> are monotonic as seen by any vCPU set PVCLOCK_TSC_STABLE_BIT, which then
>> enables usage of VDSO on Linux.  IIUC, this is similar to how it's
>> implemented on KVM.
>>
>> Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
>> ---
>> Cc: Keir Fraser <keir@xen.org>
>> Cc: Jan Beulich <jbeulich@suse.com>
>> Cc: Andrew Cooper <andrew.cooper3@citrix.com>
>> ---
>>  xen/arch/x86/time.c | 12 ++++++++++--
>>  1 file changed, 10 insertions(+), 2 deletions(-)
>>
>> diff --git a/xen/arch/x86/time.c b/xen/arch/x86/time.c
>> index 89c35d0..a17529c 100644
>> --- a/xen/arch/x86/time.c
>> +++ b/xen/arch/x86/time.c
>> @@ -917,6 +917,8 @@ static void __update_vcpu_system_time(struct vcpu *v, int force)
>>  
>>      _u.tsc_timestamp = tsc_stamp;
>>      _u.system_time   = t->stime_local_stamp;
>> +    if ( clocksource_is_tsc )
>> +        _u.flags    |= PVCLOCK_TSC_STABLE_BIT;
>>  
>>      if ( is_hvm_domain(d) )
>>          _u.tsc_timestamp += v->arch.hvm_vcpu.cache_tsc_offset;
>> @@ -1377,9 +1379,12 @@ static void time_calibration_std_rendezvous(void *_r)
>>  
>>      if ( smp_processor_id() == 0 )
>>      {
>> +        u64 last_counter;
> 
> Blank line here please.
> 
>>          while ( atomic_read(&r->semaphore) != (total_cpus - 1) )
>>              cpu_relax();
>> -        r->master_stime = read_platform_stime();
>> +        r->master_stime = read_platform_stime(&last_counter);
>> +        if ( clocksource_is_tsc )
>> +            r->master_tsc_stamp = last_counter;
>>          mb(); /* write r->master_stime /then/ signal */
>>          atomic_inc(&r->semaphore);
>>      }
>> @@ -1391,7 +1396,10 @@ static void time_calibration_std_rendezvous(void *_r)
>>          mb(); /* receive signal /then/ read r->master_stime */
>>      }
>>  
>> -    c->local_tsc_stamp = rdtsc();
>> +    if ( clocksource_is_tsc )
>> +        c->local_tsc_stamp = r->master_tsc_stamp;
>> +    else
>> +        c->local_tsc_stamp = rdtsc();
>>      c->stime_local_stamp = get_s_time();
>>      c->stime_master_stamp = r->master_stime;
>>  
> 
> The point of the rendezvous is to run rdtsc() at a the time on each cpu
> at the same time.  With this logic, it seems that you don't need the
> rendezvous at all.
> 
> Avoiding the time_calibration_std_rendezvous() entirely in this
> situation would be the better, surely?
Indeed, and would look cleaner too. I've changed the approach in this patch for
v2 following your guideline, along with some retesting.

> 
> ~Andrew
> 

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