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;
> +}
> }
> }
>
>
