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From: Peter Xu <peterx@redhat.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Hyman <huangy81@chinatelecom.cn>,
	Keqian Zhu <zhukeqian1@huawei.com>,
	qemu-devel@nongnu.org,
	"Dr . David Alan Gilbert" <dgilbert@redhat.com>
Subject: Re: [PATCH v5 09/10] KVM: Disable manual dirty log when dirty ring enabled
Date: Mon, 22 Mar 2021 12:21:44 -0400	[thread overview]
Message-ID: <20210322162144.GB16645@xz-x1> (raw)
In-Reply-To: <47bfae75-9e2e-50da-a944-a45f64f41514@redhat.com>

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



  reply	other threads:[~2021-03-22 16:28 UTC|newest]

Thread overview: 29+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-03-10 20:32 [PATCH v5 00/10] KVM: Dirty ring support (QEMU part) Peter Xu
2021-03-10 20:32 ` [PATCH v5 01/10] memory: Introduce log_sync_global() to memory listener Peter Xu
2021-03-10 20:32 ` [PATCH v5 02/10] KVM: Use a big lock to replace per-kml slots_lock Peter Xu
2021-03-22 10:47   ` Keqian Zhu
2021-03-22 13:54     ` Paolo Bonzini
2021-03-22 16:27       ` Peter Xu
2021-03-24 18:08         ` Peter Xu
2021-03-10 20:32 ` [PATCH v5 03/10] KVM: Create the KVMSlot dirty bitmap on flag changes Peter Xu
2021-03-10 20:32 ` [PATCH v5 04/10] KVM: Provide helper to get kvm dirty log Peter Xu
2021-03-10 20:32 ` [PATCH v5 05/10] KVM: Provide helper to sync dirty bitmap from slot to ramblock Peter Xu
2021-03-10 20:32 ` [PATCH v5 06/10] KVM: Simplify dirty log sync in kvm_set_phys_mem Peter Xu
2021-03-10 20:32 ` [PATCH v5 07/10] KVM: Cache kvm slot dirty bitmap size Peter Xu
2021-03-10 20:32 ` [PATCH v5 08/10] KVM: Add dirty-gfn-count property Peter Xu
2021-03-10 20:33 ` [PATCH v5 09/10] KVM: Disable manual dirty log when dirty ring enabled Peter Xu
2021-03-22  9:17   ` Keqian Zhu
2021-03-22 13:55     ` Paolo Bonzini
2021-03-22 16:21       ` Peter Xu [this message]
2021-03-10 20:33 ` [PATCH v5 10/10] KVM: Dirty ring support Peter Xu
2021-03-22 13:37   ` Keqian Zhu
2021-03-22 18:52     ` Peter Xu
2021-03-23  1:25       ` Keqian Zhu
2021-03-19 18:12 ` [PATCH v5 00/10] KVM: Dirty ring support (QEMU part) Peter Xu
2021-03-22 14:02 ` Keqian Zhu
2021-03-22 19:45   ` Peter Xu
2021-03-23  6:40     ` Keqian Zhu
2021-03-23 14:34       ` Peter Xu
2021-03-24  2:56         ` Keqian Zhu
2021-03-24 15:09           ` Peter Xu
2021-03-25  1:21             ` Keqian Zhu

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