On Sat, Mar 19, 2022 at 08:52:56AM +0100, Paolo Bonzini wrote: > On 3/18/22 19:39, Oliver Upton wrote: > > Hi Paolo, > > > > On Wed, Mar 16, 2022 at 08:47:59AM +0100, Paolo Bonzini wrote: > > > On 3/16/22 05:53, Oliver Upton wrote: > > > > The VMM has control of both the guest's TSC scale and offset. Extend the > > > > described migration algorithm in the KVM_VCPU_TSC_OFFSET documentation > > > > to cover TSC scaling. > > > > > > > > Reported-by: David Woodhouse<dwmw@xxxxxxxxxxxx> > > > > Signed-off-by: Oliver Upton<oupton@xxxxxxxxxx> > > > > --- > > > > > > > > Applies to kvm/queue (references KVM_{GET,SET}_TSC_KHZ on a VM fd). > > > > > > A few more things that have to be changed: > > > > > > > 1. Invoke the KVM_GET_CLOCK ioctl to record the host TSC (tsc_src), > > > > kvmclock nanoseconds (guest_src), and host CLOCK_REALTIME nanoseconds > > > > (host_src). > > > > > > > > > > One of two changes: > > > > > > a) Add "Multiply tsc_src by guest_freq / src_freq to obtain scaled_tsc_src", > > > add a new device attribute for the host TSC frequency. > > > > > > b) Add "Multiply tsc_src by src_ratio to obtain scaled_tsc_src", add a new > > > device attribute for the guest_frequency/host_frequency ratio. > > > > > > A third would be scaling the host TSC frequency in KVM_GETCLOCK, but that's > > > confusing IMO. > > > > Agreed -- I think kvmclock should remain as is. > > > > A fourth would be to expose the host's TSC frequency outside of KVM > > since we're really in the business of guest features, not host ones :-) > > We already have a patch that does this internally, and its visible in > > some of our open source userspace libraries [1]. > > Yeah, it was a bit of a cop out on my part but adding it to sysfs would be > nice. Please tell me if any of you going to send a patch, or even just > share it so that I can handle the upstream submission. I'll have some time to look at it next week and send upstream. I was somewhat panning if there were any other ideas here, but sysfs does seem sensible :) -- Thanks, Oliver