RE: [Xen-devel] [PATCH] replace rdtsc emulation-vs-native xen boot option with per-domain (hypervisor part)

["Zhang, Xiantao" <xiantao.zhang@xxxxxxxxx>]

Ian Pratt wrote:
>>> Well, it shouldn't be enabled by default.  That slows down all rdtsc
>>> operations for the benefit of very niche applications.  The Xen
>>> clocksource assumes that rdtsc is fast, unemulated and in need of
>>> correction. 
>>> 
>>> If someone really needs an artificial tsc, then they can enable the
>>> option for themselves.
>> 
>> While I understand and sympathize (and the idealistic side of me even
>> agrees with) your position, let me argue your point "they can enable
>> the option for themselves" by mangling a famous movie quote:
>> 
>>       "In a cloud, no-one can hear rdtsc scream"
> 
> If we had some actual bug reports detailing failures of real-world
> applications then the decision would be easy. 
> I realise it could be hard to pin strange application behaviour down
> to TSC issues, but I can't readily spot any bugs on the XenServer or
> xen.org bug trackers that might even be candidates. 

Maybe Dan can give more reasons. I think the TSC skew issue should be thre key 
one. But IMO, the TSC skew issue maybe not that important. In real-world apps, 
rdtsc is mostly read by the kernel syscall gettimeofday, and kernel can 
eliminate the skew before returning to user app through returning the value 
"plus 1 on last_tsc". Even if apps use "raw rdtsc" instruction to get timestamp 
in rare cases, I think the issue also can be solved by apps' ownself through a 
simple check.  Therefore, I still can't figure out the suasive reasons why 
adopts emulation way for the tsc read so far. 
Dan, 
    Any other necessary reasons to introduce it except fixing skew issue 
between cpus?
Xiantao
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