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From: Peter Xu <peterx@redhat.com>
To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Alex Williamson <alex.williamson@redhat.com>,
	kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
	Christophe de Dinechin <dinechin@redhat.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Sean Christopherson <sean.j.christopherson@intel.com>,
	Yan Zhao <yan.y.zhao@intel.com>, Jason Wang <jasowang@redhat.com>,
	Kevin Kevin <kevin.tian@intel.com>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	"Dr . David Alan Gilbert" <dgilbert@redhat.com>,
	Kirti Wankhede <kwankhede@nvidia.com>
Subject: Re: [PATCH v3 00/21] KVM: Dirty ring interface
Date: Thu, 9 Jan 2020 14:23:18 -0500
Message-ID: <20200109192318.GF36997@xz-x1> (raw)
In-Reply-To: <20200109140948-mutt-send-email-mst@kernel.org>

On Thu, Jan 09, 2020 at 02:13:54PM -0500, Michael S. Tsirkin wrote:
> On Thu, Jan 09, 2020 at 12:58:08PM -0500, Peter Xu wrote:
> > On Thu, Jan 09, 2020 at 09:47:11AM -0700, Alex Williamson wrote:
> > > On Thu,  9 Jan 2020 09:57:08 -0500
> > > Peter Xu <peterx@redhat.com> wrote:
> > > 
> > > > Branch is here: https://github.com/xzpeter/linux/tree/kvm-dirty-ring
> > > > (based on kvm/queue)
> > > > 
> > > > Please refer to either the previous cover letters, or documentation
> > > > update in patch 12 for the big picture.  Previous posts:
> > > > 
> > > > V1: https://lore.kernel.org/kvm/20191129213505.18472-1-peterx@redhat.com
> > > > V2: https://lore.kernel.org/kvm/20191221014938.58831-1-peterx@redhat.com
> > > > 
> > > > The major change in V3 is that we dropped the whole waitqueue and the
> > > > global lock. With that, we have clean per-vcpu ring and no default
> > > > ring any more.  The two kvmgt refactoring patches were also included
> > > > to show the dependency of the works.
> > > 
> > > Hi Peter,
> > 
> > Hi, Alex,
> > 
> > > 
> > > Would you recommend this style of interface for vfio dirty page
> > > tracking as well?  This mechanism seems very tuned to sparse page
> > > dirtying, how well does it handle fully dirty, or even significantly
> > > dirty regions?
> > 
> > That's truely the point why I think the dirty bitmap can still be used
> > and should be kept.  IIUC the dirty ring starts from COLO where (1)
> > dirty rate is very low, and (2) sync happens frequently.  That's a
> > perfect ground for dirty ring.  However it for sure does not mean that
> > dirty ring can solve all the issues.  As you said, I believe the full
> > dirty is another extreme in that dirty bitmap could perform better.
> > 
> > > We also don't really have "active" dirty page tracking
> > > in vfio, we simply assume that if a page is pinned or otherwise mapped
> > > that it's dirty, so I think we'd constantly be trying to re-populate
> > > the dirty ring with pages that we've seen the user consume, which
> > > doesn't seem like a good fit versus a bitmap solution.  Thanks,
> > 
> > Right, so I confess I don't know whether dirty ring is the ideal
> > solutioon for vfio either.  Actually if we're tracking by page maps or
> > pinnings, then IMHO it also means that it could be more suitable to
> > use an modified version of dirty ring buffer (as you suggested in the
> > other thread), in that we can track dirty using (addr, len) range
> > rather than a single page address.  That could be hard for KVM because
> > in KVM the page will be mostly trapped in 4K granularity in page
> > faults, and it'll also be hard to merge continuous entries with
> > previous ones because the userspace could be reading the entries (so
> > after we publish the previous 4K dirty page, we should not modify the
> > entry any more).
> 
> An easy way would be to keep a couple of entries around, not pushing
> them into the ring until later.  In fact deferring queue write until
> there's a bunch of data to be pushed is a very handy optimization.

I feel like I proposed similar thing in the other thread. :-)

> 
> When building UAPI's it makes sense to try and keep them generic
> rather than tying them to a given implementation.
> 
> That's one of the reasons I called for using something
> resembling vring_packed_desc.

But again, I just want to make sure I don't over-engineer...

I'll wait for further feedback from others for this.

Thanks,

-- 
Peter Xu


  reply index

Thread overview: 82+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-01-09 14:57 Peter Xu
2020-01-09 14:57 ` [PATCH v3 01/21] vfio: introduce vfio_iova_rw to read/write a range of IOVAs Peter Xu
2020-01-09 14:57 ` [PATCH v3 02/21] drm/i915/gvt: subsitute kvm_read/write_guest with vfio_iova_rw Peter Xu
2020-01-09 14:57 ` [PATCH v3 03/21] KVM: Remove kvm_read_guest_atomic() Peter Xu
2020-01-09 14:57 ` [PATCH v3 04/21] KVM: Add build-time error check on kvm_run size Peter Xu
2020-01-09 14:57 ` [PATCH v3 05/21] KVM: X86: Change parameter for fast_page_fault tracepoint Peter Xu
2020-01-09 14:57 ` [PATCH v3 06/21] KVM: X86: Don't take srcu lock in init_rmode_identity_map() Peter Xu
2020-01-09 14:57 ` [PATCH v3 07/21] KVM: Cache as_id in kvm_memory_slot Peter Xu
2020-01-09 14:57 ` [PATCH v3 08/21] KVM: X86: Drop x86_set_memory_region() Peter Xu
2020-01-09 14:57 ` [PATCH v3 09/21] KVM: X86: Don't track dirty for KVM_SET_[TSS_ADDR|IDENTITY_MAP_ADDR] Peter Xu
2020-01-19  9:01   ` Paolo Bonzini
2020-01-20  6:45     ` Peter Xu
2020-01-21 15:56   ` Sean Christopherson
2020-01-21 16:14     ` Paolo Bonzini
2020-01-28  5:50     ` Peter Xu
2020-01-28 18:24       ` Sean Christopherson
2020-01-31 15:08         ` Peter Xu
2020-01-31 19:33           ` Sean Christopherson
2020-01-31 20:28             ` Peter Xu
2020-01-31 20:36               ` Sean Christopherson
2020-01-31 20:55                 ` Peter Xu
2020-01-31 21:29                   ` Sean Christopherson
2020-01-31 22:16                     ` Peter Xu
2020-01-31 22:20                       ` Sean Christopherson
2020-01-09 14:57 ` [PATCH v3 10/21] KVM: Pass in kvm pointer into mark_page_dirty_in_slot() Peter Xu
2020-01-09 14:57 ` [PATCH v3 11/21] KVM: Move running VCPU from ARM to common code Peter Xu
2020-01-09 14:57 ` [PATCH v3 12/21] KVM: X86: Implement ring-based dirty memory tracking Peter Xu
2020-01-09 16:29   ` Michael S. Tsirkin
2020-01-09 16:56     ` Alex Williamson
2020-01-09 19:21       ` Peter Xu
2020-01-09 19:36         ` Michael S. Tsirkin
2020-01-09 19:15     ` Peter Xu
2020-01-09 19:35       ` Michael S. Tsirkin
2020-01-09 20:19         ` Peter Xu
2020-01-09 22:18           ` Michael S. Tsirkin
2020-01-10 15:29             ` Peter Xu
2020-01-12  6:24               ` Michael S. Tsirkin
2020-01-14 20:01         ` Peter Xu
2020-01-15  6:50           ` Michael S. Tsirkin
2020-01-15 15:20             ` Peter Xu
2020-01-19  9:09       ` Paolo Bonzini
2020-01-19 10:12         ` Michael S. Tsirkin
2020-01-20  7:29           ` Peter Xu
2020-01-20  7:47             ` Michael S. Tsirkin
2020-01-21  8:29               ` Peter Xu
2020-01-21 10:25                 ` Paolo Bonzini
2020-01-21 10:24             ` Paolo Bonzini
2020-01-11  4:49   ` kbuild test robot
2020-01-11 23:19   ` kbuild test robot
2020-01-15  6:47   ` Michael S. Tsirkin
2020-01-15 15:27     ` Peter Xu
2020-01-16  8:38   ` Michael S. Tsirkin
2020-01-16 16:27     ` Peter Xu
2020-01-17  9:50       ` Michael S. Tsirkin
2020-01-20  6:48         ` Peter Xu
2020-01-09 14:57 ` [PATCH v3 13/21] KVM: Make dirty ring exclusive to dirty bitmap log Peter Xu
2020-01-09 14:57 ` [PATCH v3 14/21] KVM: Don't allocate dirty bitmap if dirty ring is enabled Peter Xu
2020-01-09 16:41   ` Peter Xu
2020-01-09 14:57 ` [PATCH v3 15/21] KVM: selftests: Always clear dirty bitmap after iteration Peter Xu
2020-01-09 14:57 ` [PATCH v3 16/21] KVM: selftests: Sync uapi/linux/kvm.h to tools/ Peter Xu
2020-01-09 14:57 ` [PATCH v3 17/21] KVM: selftests: Use a single binary for dirty/clear log test Peter Xu
2020-01-09 14:57 ` [PATCH v3 18/21] KVM: selftests: Introduce after_vcpu_run hook for dirty " Peter Xu
2020-01-09 14:57 ` [PATCH v3 19/21] KVM: selftests: Add dirty ring buffer test Peter Xu
2020-01-09 14:57 ` [PATCH v3 20/21] KVM: selftests: Let dirty_log_test async for dirty ring test Peter Xu
2020-01-09 14:57 ` [PATCH v3 21/21] KVM: selftests: Add "-c" parameter to dirty log test Peter Xu
2020-01-09 15:59 ` [PATCH v3 00/21] KVM: Dirty ring interface Michael S. Tsirkin
2020-01-09 16:17   ` Peter Xu
2020-01-09 16:40     ` Michael S. Tsirkin
2020-01-09 17:08       ` Peter Xu
2020-01-09 19:08         ` Michael S. Tsirkin
2020-01-09 19:39           ` Peter Xu
2020-01-09 20:42             ` Paolo Bonzini
2020-01-09 22:28             ` Michael S. Tsirkin
2020-01-10 15:10               ` Peter Xu
2020-01-09 16:47 ` Alex Williamson
2020-01-09 17:58   ` Peter Xu
2020-01-09 19:13     ` Michael S. Tsirkin
2020-01-09 19:23       ` Peter Xu [this message]
2020-01-09 19:37         ` Michael S. Tsirkin
2020-01-09 20:51       ` Paolo Bonzini
2020-01-09 22:21         ` Michael S. Tsirkin
2020-01-19  9:11 ` Paolo Bonzini

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