Hi Peter, On 2021/3/11 4:33, Peter Xu wrote: > KVM_CAP_MANUAL_DIRTY_LOG_PROTECT2 is for KVM_CLEAR_DIRTY_LOG, which is only > useful for KVM_GET_DIRTY_LOG. Skip enabling it for kvm dirty ring. > > More importantly, KVM_DIRTY_LOG_INITIALLY_SET will not wr-protect all the > pages > initially, which is against how kvm dirty ring is used - there's no way for > kvm > dirty ring to re-protect a page before it's notified as being written first > with a GFN entry in the ring! So when KVM_DIRTY_LOG_INITIALLY_SET is enabled > with dirty ring, we'll see silent data loss after migration. I feel a little regret that dirty ring can not work with KVM_DIRTY_LOG_INITIALLY_SET ... With KVM_DIRTY_LOG_INITIALLY_SET, we can speedup dirty log start. More important, we can enable dirty log gradually. For write fault based dirty log, it greatly reduces the side effect of dirty log over guest.
I hope we can put forward another similar optimization under dirty ring mode. :) Thanks, Keqian > > Signed-off-by: Peter Xu <pet...@redhat.com> > --- > accel/kvm/kvm-all.c | 37 +++++++++++++++++++++++-------------- > 1 file changed, 23 insertions(+), 14 deletions(-) > > diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c > index 10137b6af11..ae9393266b2 100644 > --- a/accel/kvm/kvm-all.c > +++ b/accel/kvm/kvm-all.c > @@ -2173,20 +2173,29 @@ static int kvm_init(MachineState *ms) > } > } > > - dirty_log_manual_caps = > - kvm_check_extension(s, KVM_CAP_MANUAL_DIRTY_LOG_PROTECT2); > - dirty_log_manual_caps &= (KVM_DIRTY_LOG_MANUAL_PROTECT_ENABLE | > - KVM_DIRTY_LOG_INITIALLY_SET); > - s->manual_dirty_log_protect = dirty_log_manual_caps; > - if (dirty_log_manual_caps) { > - ret = kvm_vm_enable_cap(s, KVM_CAP_MANUAL_DIRTY_LOG_PROTECT2, 0, > - dirty_log_manual_caps); > - if (ret) { > - warn_report("Trying to enable capability %"PRIu64" of " > - "KVM_CAP_MANUAL_DIRTY_LOG_PROTECT2 but failed. " > - "Falling back to the legacy mode. ", > - dirty_log_manual_caps); > - s->manual_dirty_log_protect = 0; > + /* > + * KVM_CAP_MANUAL_DIRTY_LOG_PROTECT2 is not needed when dirty ring is > + * enabled. More importantly, KVM_DIRTY_LOG_INITIALLY_SET will assume no > + * page is wr-protected initially, which is against how kvm dirty ring is > + * usage - kvm dirty ring requires all pages are wr-protected at the very > + * beginning. Enabling this feature for dirty ring causes data > corruption. > + */ > + if (!s->kvm_dirty_ring_enabled) { > + dirty_log_manual_caps = > + kvm_check_extension(s, KVM_CAP_MANUAL_DIRTY_LOG_PROTECT2); > + dirty_log_manual_caps &= (KVM_DIRTY_LOG_MANUAL_PROTECT_ENABLE | > + KVM_DIRTY_LOG_INITIALLY_SET); > + s->manual_dirty_log_protect = dirty_log_manual_caps; > + if (dirty_log_manual_caps) { > + ret = kvm_vm_enable_cap(s, KVM_CAP_MANUAL_DIRTY_LOG_PROTECT2, 0, > + dirty_log_manual_caps); > + if (ret) { > + warn_report("Trying to enable capability %"PRIu64" of " > + "KVM_CAP_MANUAL_DIRTY_LOG_PROTECT2 but failed. " > + "Falling back to the legacy mode. ", > + dirty_log_manual_caps); > + s->manual_dirty_log_protect = 0; > + } > } > } > >