On 14/03/2019 14:50, Olaf Hering wrote: > Am Wed, 13 Mar 2019 03:18:39 -0600 > schrieb "Jan Beulich" <[email protected]>: > >> I'm sorry, but I continue to object to this adjustment getting done >> both by default _and_ not in a per-guest manner. As said before, >> you can't demand guests to run NTP, and hence you can't expect >> them to get along with a few hundred kHz jump in observed TSC >> frequency. Whether the performance drop due to vTSC use is >> better or worse is a policy decision, which we should leave to the >> admin. Hence the feature needs to be off by default, and there >> needs to be at least a host-wide control to enable it; a per-guest >> control would be better. IOW I explicitly do not agree with the >> last sentence of the commit message. > > So this seems the be the essential part that prevents moving forward. > > Your claim is basically that "we do not know how the workload reacts > to frequency change". > My claim is basically "there is enough evidence that syncing with > external clock is required if the frequency remotely matters". > > I think that conflict can not be easily solved. > > One way to solve it would be a knob that injects a value into the > proposed "vtsc_tolerance_khz" variable, leave the calculation to > the host admin, and leave code in tsc_set_info basically as is. > > Maybe "xl set-params" can be the way to change the value, that way > it can be changed globally at runtime if needed. > > In staging the change would affect HVM and PVH. I never ran PVH, > I have to assume it behaves like HVM in this regard.
I think Andrew (or Ian?) once suggested to handle this whole mess in the migration stream instead of the hypervisor. So why don't you: - add a domain config item for specifying the allowed jitter - add that value to the migration struct xc_sr_rec_tsc_info (there is a reserved field available) - and then modify handle_tsc_info() in tools/libxc/xc_sr_common_x86.c to test the host frequency to be in the acceptable range and if this is the case put the host frequency into the gtsc_khz parameter of the xc_domain_set_tsc_info() call This would be the least intrusive change allowing maximum flexibility IMO. Juergen _______________________________________________ Xen-devel mailing list [email protected] https://lists.xenproject.org/mailman/listinfo/xen-devel
