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

From: H. Peter Anvin
Date: Thu Oct 23 2008 - 20:46:53 EST

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.

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