On 03/05/2017 15:40, Marcelo Tosatti wrote: >>> In the masterclock enabled case, kvmclock_offset must be adjusted so >>> that user_ns.clock = master_kernel_ns + kvmclock_offset (that is, the >>> value set from KVM_SET_CLOCK is the one visible at system_timestamp). >>> >>> This way the guest clock: >>> >>> 1. Starts counting when KVM_SET_CLOCK executes. >>> 2. With the value provided by userspace. >> >> So this fixes rounding errors? > > No. It just does the correct > > "user_ns.clock = master_kernel_ns + kvmclock_offset = return value of > get_kvmclock_ns(kvm) at moment of KVM_SET_CLOCK" > > (that is, when guest_rdtsc() == tsc_timestamp, so guest_rdtsc() - tsc_timestamp = 0, > you want get_kvmclock_ns() to return user_ns.clock). Yep, and it currently doesn't match because the current code introduces rounding errors? >>> + kvm_for_each_vcpu(i, vcpu, kvm) >>> + kvm_make_request(KVM_REQ_CLOCK_UPDATE, vcpu); >> kvm_gen_update_masterclock already does that, why did you move that to >> before the assignment? > Its after the assignment of kvmclock_offset because > KVM_REQ_CLOCK_UPDATES processing by vcpus make use of kvmclock_offset. Sure, but why did you move kvm_gen_update_masterclock before? If you left kvm_gen_update_masterclock after the update of kvm->arch.kvmclock_offset, the duplicate KVM_REQ_CLOCK_UPDATE would not be necessary. Paolo > So if you change that value, you have to request the vcpus to > recalculate their kvmclock areas using the new kvmclock_offset.
