On 10/09/2012 04:24 PM, Marcelo Tosatti wrote: > On Tue, Oct 09, 2012 at 02:12:18PM +0200, Avi Kivity wrote: >> On 10/08/2012 07:30 PM, Marcelo Tosatti wrote: >> > >> > From Intel's manual: >> > >> > • If an execution of WRMSR to the IA32_TIME_STAMP_COUNTER MSR adds (or >> > subtracts) value X from the TSC, >> > the logical processor also adds (or subtracts) value X from the >> > IA32_TSC_ADJUST MSR. >> > >> > This is not handled in the patch. >> > >> > To support migration, it will be necessary to differentiate between >> > guest initiated and userspace-model initiated msr write. That is, >> > only guest initiated TSC writes should affect the value of >> > IA32_TSC_ADJUST MSR. >> > >> > Avi, any better idea? >> > >> >> I think we need that anyway, since there are some read-only MSRs that >> need to be configured by the host (nvmx capabilities). So if we add >> that feature it will be useful elsewhere. I don't think it's possible >> to do it in any other way: >> >> "Local offset value of the IA32_TSC for a >> logical processor. Reset value is Zero. A >> write to IA32_TSC will modify the local >> offset in IA32_TSC_ADJUST and the >> content of IA32_TSC, but does not affect >> the internal invariant TSC hardware." >> >> What we want to do is affect the internal invariant TSC hardware, so we >> can't do that through the normal means. >> >> btw, will tsc writes from userspace (after live migration) cause tsc >> skew? If so we should think how to model a guest-wide tsc. > > No because there is an easy shortcut: > > if (level == KVM_PUT_FULL_STATE) { > /* > * KVM is yet unable to synchronize TSC values of multiple VCPUs > * on > * writeback. Until this is fixed, we only write the offset to > * SMP > * guests after migration, desynchronizing the VCPUs, but > * avoiding > * huge jump-backs that would occur without any writeback at > * all. > */ > if (smp_cpus == 1 || env->tsc != 0) { > kvm_msr_entry_set(&msrs[n++], MSR_IA32_TSC, env->tsc); > } > } Still we write back after migration. So this needs to be fixed (or I misunderstood you). -- error compiling committee.c: too many arguments to function -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html