Re: [PATCH] Skip tsc synchronization checks if CONSTANT_TSC bit is set.

["H. Peter Anvin" <hpa@zytor.com>]

Andi Kleen wrote:
>> As far as skipping the check, it makes sense for me in the case of known 
>> virtualization platforms; a CPU feature bit, real or synthetic, is a 
>> very clean way to do that. 
> 
> I don't think adding detection for non PV Hypervisors is anywhere clean
> Even if it's only VMware today, tomorrow it will be a few more
> and long term you might need to support all of the obscuro hypervisors
> that are out there. Just seems like a slippery slope. Either it's 
> paravirtual or it's not, but it should attempt to be both. If the hypervisor
> doesn't emulate TSC well enough that the native code works it's entirely
> reasonable to let it use some other timer, like it has been always
> done in the past.

That is at least to some degree nonsense, simply because we are all well 
down that particular "slippery slope": we have hardware blacklists and 
whitelists, CPU-specific workarounds, and so on all over the place, and 
in that sense a hypervisor really isn't different than another hardware 
platform.

	-hpa