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

[Jan Beulich <jbeulich@xxxxxxxx>]

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?

Jan

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