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

[Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>]

On 15/01/2020 12:25, Igor Druzhinin wrote:
> 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?
> 

Ignore my reply. I was thinking you're asking whether the original code
would do such a thing. Although I'm concerned if what you say actually
applies to the original code as well. Would you think the existing logic
handles it already?

Igor

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