On Fri, Oct 23, 2015 at 08:35:20AM -0200, Marcelo Tosatti wrote: > On Thu, Oct 22, 2015 at 04:45:21PM -0200, Eduardo Habkost wrote: > > On Tue, Oct 20, 2015 at 03:22:51PM +0800, Haozhong Zhang wrote: > > > This patchset enables QEMU to save/restore vcpu's TSC rate during the > > > migration. When cooperating with KVM which supports TSC scaling, guest > > > programs can observe a consistent guest TSC rate even though they are > > > migrated among machines with different host TSC rates. > > > > > > A pair of cpu options 'save-tsc-freq' and 'load-tsc-freq' are added to > > > control the migration of vcpu's TSC rate. > > > > The requirements and goals aren't clear to me. I see two possible use > > cases, here: > > > > 1) Best effort to keep TSC frequency constant if possible (but not > > aborting migration if not possible). This would be an interesting > > default, but a bit unpredictable. > > 2) Strictly ensuring TSC frequency stays constant on migration (and > > aborting migration if not possible). This would be an useful feature, > > but can't be enabled by default unless both hosts have the same TSC > > frequency or support TSC scaling. > > Only destination needs to support TSC scaling, to match the frequency > of the incoming host. True. > > The KVM code for this feature has submitted or integrated? > > > Which one(s) you are trying to implement? > > > > In other words, what is the right behavior when KVM_SET_TSC_KHZ fails or > > KVM_CAP_TSC_CONTROL is not available? We can't answer that question if > > the requirements and goals are not clear. > > > > Once we know what exactly is the goal, we could enable the new mode with > > a single option, instead of raw options to control migration stream > > loading/saving. > > Windows and Linux guests have paravirt clocks and/or options to > disable direct TSC usage for timekeeping purposes. So disabling > migration seems overkill. I assume that users who set TSC frequency explicitly in the VM config care about it (otherwise they wouldn't be setting it explicitly) and don't want it to change after migration. -- Eduardo -- 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
