Re: [PATCH v5 09/10] KVM: Disable manual dirty log when dirty ring enabled
On Mon, Mar 22, 2021 at 02:55:44PM +0100, Paolo Bonzini wrote: > On 22/03/21 10:17, Keqian Zhu wrote: > > 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. :) > > Indeed, perhaps (even though KVM_GET_DIRTY_LOG does not make sense with > dirty ring) we could allow KVM_CLEAR_DIRTY_LOG. Right, KVM_CLEAR_DIRTY_LOG is a good interface to reuse so as to grant userspace more flexibility to explicit wr-protect some guest pages. However that'll need kernel reworks - obviously when I worked on the kernel part I didn't notice this issue.. To make it a complete work, IMHO we'll also need QEMU to completely drop the whole dirty bitmap in all the layers, then I'd expect the dirty ring idea as a whole start to make more difference on huge vms. They all just need some more work which should be based on this series. Shall we make it a "TODO" though? E.g., I can add a comment here mentioning about this issue. I still hope we can have the qemu series lands soon first, since it'll be bigger project to fully complete it. Paolo? Thanks, -- Peter Xu
Re: [PATCH v5 09/10] KVM: Disable manual dirty log when dirty ring enabled
On 22/03/21 10:17, Keqian Zhu wrote: 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. :) Indeed, perhaps (even though KVM_GET_DIRTY_LOG does not make sense with dirty ring) we could allow KVM_CLEAR_DIRTY_LOG. Paolo
Re: [PATCH v5 09/10] KVM: Disable manual dirty log when dirty ring enabled
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 > --- > 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; > +} > } > } > >
[PATCH v5 09/10] KVM: Disable manual dirty log when dirty ring enabled
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. Signed-off-by: Peter Xu --- 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; +} } } -- 2.26.2