Re: [Xen-devel] [PATCH] x86/time: update TSC stamp on restore from deep C-state

[Igor Druzhinin <igor.druzhinin@citrix.com>]

On 15/01/2020 11:40, Jan Beulich wrote:
> On 15.01.2020 10:47, Roger Pau Monné wrote:
>> On Tue, Jan 14, 2020 at 07:36:21PM +0000, Igor Druzhinin wrote:
>>> --- a/xen/arch/x86/time.c
>>> +++ b/xen/arch/x86/time.c
>>> @@ -955,10 +955,16 @@ u64 stime2tsc(s_time_t stime)
>>>  
>>>  void cstate_restore_tsc(void)
>>>  {
>>> +    struct cpu_time *t = &this_cpu(cpu_time);
>>> +
>>>      if ( boot_cpu_has(X86_FEATURE_NONSTOP_TSC) )
>>>          return;
>>>  
>>> -    write_tsc(stime2tsc(read_platform_stime(NULL)));
>>> +    t->stamp.master_stime = read_platform_stime(NULL);
>>> +    t->stamp.local_tsc = stime2tsc(t->stamp.master_stime);
>>> +    t->stamp.local_stime = t->stamp.master_stime;
>>> +
>>> +    write_tsc(t->stamp.local_tsc);
>>
>> In order to avoid the TSC write (and the likely associated vmexit),
>> could you instead do:
>>
>> t->stamp.local_stime = t->stamp.master_stime = read_platform_stime(NULL);
>> t->stamp.local_tsc = rdtsc_ordered();
>>
>> I think it should achieve the same as it syncs the local TSC stamp and
>> times, would avoid the TSC write and slightly simplifies the logic.
> 
> Wouldn't this result in guests possibly observing the TSC moving
> backwards?

Yes, I think so. Would restoring from TSC stamp if it's higher than
platform time better you think?

Igor

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