Messages in this thread | | | Date | Tue, 30 Sep 2008 00:46:31 +0200 | From | Gerd Hoffmann <> | Subject | Re: Use CPUID to communicate with the hypervisor. |
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Alok Kataria wrote: > On Mon, 2008-09-29 at 11:46 -0700, Gerd Hoffmann wrote: >> But even that you can't take for granted, see the >> discussion of the "tsc-may-change-on-migration" problem. > > I may have been unclear in my first attempt to this question, let me try > again. > If the frequency of tsc changes during migration, it should be the task > of hypervisor to handle it. There could be multiple ways to solve that > problem, either the hypervisor emulates the old frequency (by whatever > way) or there are cpufreq drivers in the guest which detect changes in > frequency, and ask the hypervisor for the new frequency. The interface > still allows you to query the cpuid leaf and get the new frequency. > right ?
This small print is part of the guest/host ABI though, so hypervisors must agree here too, be it "tsc is constant" or "re-read tsc freq on $event" or whatever else. Otherwise it isn't a generic interface.
>> The real big problem are other closed-source hypervisors (VirtualPC / >> Hyper-V / Parallels / ...). How can we be sure they don't define that >> leaf to something different? > > How does that matter, if we are able to standardize all this then, > hypervisors which want to run a Linux guest should effectively play by > the standards over here or else they would never work properly on Linux.
Although we are working on world domination I think we are not close enough yet that this is a realistic point of view.
> Hmm, I am confused, from the patch i posted above, in > native_calibrate_tsc > > + tsc_khz = hypervisor_tsc_freq(); > + if (tsc_khz) > + return tsc_khz; > > We do ignore zero values over here.
Oh, ok.
I expected the check explicitly coded within the hypervisor_tsc_freq() function. This deserves at least a comment saying that this side effect is actually intentional.
cheers, Gerd
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