lkml.org 
[lkml]   [2020]   [Dec]   [8]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [PATCH v2 1/3] KVM: x86: implement KVM_{GET|SET}_TSC_STATE
    On Sun, Dec 06, 2020 at 05:19:16PM +0100, Thomas Gleixner wrote:
    > On Thu, Dec 03 2020 at 19:11, Maxim Levitsky wrote:
    > > + case KVM_SET_TSC_STATE: {
    > > + struct kvm_tsc_state __user *user_tsc_state = argp;
    > > + struct kvm_tsc_state tsc_state;
    > > + u64 host_tsc, wall_nsec;
    > > +
    > > + u64 new_guest_tsc, new_guest_tsc_offset;
    > > +
    > > + r = -EFAULT;
    > > + if (copy_from_user(&tsc_state, user_tsc_state, sizeof(tsc_state)))
    > > + goto out;
    > > +
    > > + kvm_get_walltime(&wall_nsec, &host_tsc);
    > > + new_guest_tsc = tsc_state.tsc;
    > > +
    > > + if (tsc_state.flags & KVM_TSC_STATE_TIMESTAMP_VALID) {
    > > + s64 diff = wall_nsec - tsc_state.nsec;
    > > + if (diff >= 0)
    > > + new_guest_tsc += nsec_to_cycles(vcpu, diff);
    > > + else
    > > + new_guest_tsc -= nsec_to_cycles(vcpu, -diff);
    > > + }
    > > +
    > > + new_guest_tsc_offset = new_guest_tsc - kvm_scale_tsc(vcpu, host_tsc);
    > > + kvm_vcpu_write_tsc_offset(vcpu, new_guest_tsc_offset);
    >
    > >From a timekeeping POV and the guests expectation of TSC this is
    > fundamentally wrong:
    >
    > tscguest = scaled(hosttsc) + offset
    >
    > The TSC has to be viewed systemwide and not per CPU. It's systemwide
    > used for timekeeping and for that to work it has to be synchronized.
    >
    > Why would this be different on virt? Just because it's virt or what?
    >
    > Migration is a guest wide thing and you're not migrating single vCPUs.
    >
    > This hackery just papers over he underlying design fail that KVM looks
    > at the TSC per vCPU which is the root cause and that needs to be fixed.

    It already does it: The unified TSC offset is kept at kvm->arch.cur_tsc_offset.


    \
     
     \ /
      Last update: 2020-12-08 15:26    [W:3.905 / U:0.012 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site