From: "Michael S. Tsirkin" <mst@redhat.com>
To: Peter Xu <peterx@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>,
linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
Sean Christopherson <sean.j.christopherson@intel.com>,
"Dr . David Alan Gilbert" <dgilbert@redhat.com>,
Vitaly Kuznetsov <vkuznets@redhat.com>
Subject: Re: [PATCH RFC 04/15] KVM: Implement ring-based dirty memory tracking
Date: Mon, 16 Dec 2019 04:47:36 -0500 [thread overview]
Message-ID: <20191216044619-mutt-send-email-mst@kernel.org> (raw)
In-Reply-To: <20191215173302.GB83861@xz-x1>
On Sun, Dec 15, 2019 at 12:33:02PM -0500, Peter Xu wrote:
> On Thu, Dec 12, 2019 at 01:08:14AM +0100, Paolo Bonzini wrote:
> > >>> What depends on what here? Looks suspicious ...
> > >>
> > >> Hmm, I think maybe it can be removed because the entry pointer
> > >> reference below should be an ordering constraint already?
> >
> > entry->xxx depends on ring->reset_index.
>
> Yes that's true, but...
>
> entry = &ring->dirty_gfns[ring->reset_index & (ring->size - 1)];
> /* barrier? */
> next_slot = READ_ONCE(entry->slot);
> next_offset = READ_ONCE(entry->offset);
>
> ... I think entry->xxx depends on entry first, then entry depends on
> reset_index. So it seems fine because all things have a dependency?
Is reset_index changed from another thread then?
If yes then you want to read reset_index with READ_ONCE.
That includes a dependency barrier.
> >
> > >>> what's the story around locking here? Why is it safe
> > >>> not to take the lock sometimes?
> > >>
> > >> kvm_dirty_ring_push() will be with lock==true only when the per-vm
> > >> ring is used. For per-vcpu ring, because that will only happen with
> > >> the vcpu context, then we don't need locks (so kvm_dirty_ring_push()
> > >> is called with lock==false).
> >
> > FWIW this will be done much more nicely in v2.
> >
> > >>>> + page = alloc_page(GFP_KERNEL | __GFP_ZERO);
> > >>>> + if (!page) {
> > >>>> + r = -ENOMEM;
> > >>>> + goto out_err_alloc_page;
> > >>>> + }
> > >>>> + kvm->vm_run = page_address(page);
> > >>>
> > >>> So 4K with just 8 bytes used. Not as bad as 1/2Mbyte for the ring but
> > >>> still. What is wrong with just a pointer and calling put_user?
> > >>
> > >> I want to make it the start point for sharing fields between
> > >> user/kernel per-vm. Just like kvm_run for per-vcpu.
> >
> > This page is actually not needed at all. Userspace can just map at
> > KVM_DIRTY_LOG_PAGE_OFFSET, the indices reside there. You can drop
> > kvm_vm_run completely.
>
> I changed it because otherwise we use one entry of the padding, and
> all the rest of paddings are a waste of memory because we can never
> really use the padding as new fields only for the 1st entry which
> overlaps with the indices. IMHO that could even waste more than 4k.
>
> (for now we only "waste" 4K for per-vm, kvm_run is already mapped so
> no waste there, not to say potentially I still think we can use the
> kvm_vm_run in the future)
>
> >
> > >>>> + } else {
> > >>>> + /*
> > >>>> + * Put onto per vm ring because no vcpu context. Kick
> > >>>> + * vcpu0 if ring is full.
> > >>>
> > >>> What about tasks on vcpu 0? Do guests realize it's a bad idea to put
> > >>> critical tasks there, they will be penalized disproportionally?
> > >>
> > >> Reasonable question. So far we can't avoid it because vcpu exit is
> > >> the event mechanism to say "hey please collect dirty bits". Maybe
> > >> someway is better than this, but I'll need to rethink all these
> > >> over...
> > >
> > > Maybe signal an eventfd, and let userspace worry about deciding what to
> > > do.
> >
> > This has to be done synchronously. But the vm ring should be used very
> > rarely (it's for things like kvmclock updates that write to guest memory
> > outside a vCPU), possibly a handful of times in the whole run of the VM.
>
> I've summarized a list of callers who might dirty guest memory in the
> other thread, it seems to me that even the kvm clock is using per-vcpu
> contexts.
>
> >
> > >>> KVM_DIRTY_RING_MAX_ENTRIES is not part of UAPI.
> > >>> So how does userspace know what's legal?
> > >>> Do you expect it to just try?
> > >>
> > >> Yep that's what I thought. :)
> >
> > We should return it for KVM_CHECK_EXTENSION.
>
> OK. I'll drop the versioning.
>
> --
> Peter Xu
next prev parent reply other threads:[~2019-12-16 9:47 UTC|newest]
Thread overview: 121+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-11-29 21:34 [PATCH RFC 00/15] KVM: Dirty ring interface Peter Xu
2019-11-29 21:34 ` [PATCH RFC 01/15] KVM: Move running VCPU from ARM to common code Peter Xu
2019-12-03 19:01 ` Sean Christopherson
2019-12-04 9:42 ` Paolo Bonzini
2019-12-09 22:05 ` Peter Xu
2019-11-29 21:34 ` [PATCH RFC 02/15] KVM: Add kvm/vcpu argument to mark_dirty_page_in_slot Peter Xu
2019-12-02 19:32 ` Sean Christopherson
2019-12-02 20:49 ` Peter Xu
2019-11-29 21:34 ` [PATCH RFC 03/15] KVM: Add build-time error check on kvm_run size Peter Xu
2019-12-02 19:30 ` Sean Christopherson
2019-12-02 20:53 ` Peter Xu
2019-12-02 22:19 ` Sean Christopherson
2019-12-02 22:40 ` Peter Xu
2019-12-03 5:50 ` Sean Christopherson
2019-12-03 13:41 ` Paolo Bonzini
2019-12-03 17:04 ` Peter Xu
2019-11-29 21:34 ` [PATCH RFC 04/15] KVM: Implement ring-based dirty memory tracking Peter Xu
2019-12-02 20:10 ` Sean Christopherson
2019-12-02 21:16 ` Peter Xu
2019-12-02 21:50 ` Sean Christopherson
2019-12-02 23:09 ` Peter Xu
2019-12-03 13:48 ` Paolo Bonzini
2019-12-03 18:46 ` Sean Christopherson
2019-12-04 10:05 ` Paolo Bonzini
2019-12-07 0:29 ` Sean Christopherson
2019-12-09 9:37 ` Paolo Bonzini
2019-12-09 21:54 ` Peter Xu
2019-12-10 10:07 ` Paolo Bonzini
2019-12-10 15:52 ` Peter Xu
2019-12-10 17:09 ` Paolo Bonzini
2019-12-15 17:21 ` Peter Xu
2019-12-16 10:08 ` Paolo Bonzini
2019-12-16 18:54 ` Peter Xu
2019-12-17 9:01 ` Paolo Bonzini
2019-12-17 16:24 ` Peter Xu
2019-12-17 16:28 ` Paolo Bonzini
2019-12-18 21:58 ` Peter Xu
2019-12-18 22:24 ` Sean Christopherson
2019-12-18 22:37 ` Paolo Bonzini
2019-12-18 22:49 ` Peter Xu
2019-12-17 2:28 ` Tian, Kevin
2019-12-17 16:18 ` Alex Williamson
2019-12-17 16:30 ` Paolo Bonzini
2019-12-18 0:29 ` Tian, Kevin
[not found] ` <AADFC41AFE54684AB9EE6CBC0274A5D19D645E5F@SHSMSX104.ccr.corp.intel.com>
2019-12-17 5:17 ` Tian, Kevin
2019-12-17 5:25 ` Yan Zhao
2019-12-17 16:24 ` Alex Williamson
2019-12-03 19:13 ` Sean Christopherson
2019-12-04 10:14 ` Paolo Bonzini
2019-12-04 14:33 ` Sean Christopherson
2019-12-04 10:38 ` Jason Wang
2019-12-04 11:04 ` Paolo Bonzini
2019-12-04 19:52 ` Peter Xu
2019-12-05 6:51 ` Jason Wang
2019-12-05 12:08 ` Peter Xu
2019-12-05 13:12 ` Jason Wang
2019-12-10 13:25 ` Michael S. Tsirkin
2019-12-10 13:31 ` Paolo Bonzini
2019-12-10 16:02 ` Peter Xu
2019-12-10 21:53 ` Michael S. Tsirkin
2019-12-11 9:05 ` Paolo Bonzini
2019-12-11 13:04 ` Michael S. Tsirkin
2019-12-11 14:54 ` Peter Xu
2019-12-10 21:48 ` Michael S. Tsirkin
2019-12-11 12:53 ` Michael S. Tsirkin
2019-12-11 14:14 ` Paolo Bonzini
2019-12-11 20:59 ` Peter Xu
2019-12-11 22:57 ` Michael S. Tsirkin
2019-12-12 0:08 ` Paolo Bonzini
2019-12-12 7:36 ` Michael S. Tsirkin
2019-12-12 8:12 ` Paolo Bonzini
2019-12-12 10:38 ` Michael S. Tsirkin
2019-12-15 17:33 ` Peter Xu
2019-12-16 9:47 ` Michael S. Tsirkin [this message]
2019-12-16 15:07 ` Peter Xu
2019-12-16 15:33 ` Michael S. Tsirkin
2019-12-16 15:47 ` Peter Xu
2019-12-11 17:24 ` Christophe de Dinechin
2019-12-13 20:23 ` Peter Xu
2019-12-14 7:57 ` Paolo Bonzini
2019-12-14 16:26 ` Peter Xu
2019-12-16 9:29 ` Paolo Bonzini
2019-12-16 15:26 ` Peter Xu
2019-12-16 15:31 ` Paolo Bonzini
2019-12-16 15:43 ` Peter Xu
2019-12-17 12:16 ` Christophe de Dinechin
2019-12-17 12:19 ` Paolo Bonzini
2019-12-17 15:38 ` Peter Xu
2019-12-17 16:31 ` Paolo Bonzini
2019-12-17 16:42 ` Peter Xu
2019-12-17 16:48 ` Paolo Bonzini
2019-12-17 19:41 ` Peter Xu
2019-12-18 0:33 ` Paolo Bonzini
2019-12-18 16:32 ` Peter Xu
2019-12-18 16:41 ` Paolo Bonzini
2019-12-20 18:19 ` Peter Xu
2019-11-29 21:34 ` [PATCH RFC 05/15] KVM: Make dirty ring exclusive to dirty bitmap log Peter Xu
2019-11-29 21:34 ` [PATCH RFC 06/15] KVM: Introduce dirty ring wait queue Peter Xu
2019-11-29 21:34 ` [PATCH RFC 07/15] KVM: X86: Implement ring-based dirty memory tracking Peter Xu
2019-11-29 21:34 ` [PATCH RFC 08/15] KVM: selftests: Always clear dirty bitmap after iteration Peter Xu
2019-11-29 21:34 ` [PATCH RFC 09/15] KVM: selftests: Sync uapi/linux/kvm.h to tools/ Peter Xu
2019-11-29 21:35 ` [PATCH RFC 10/15] KVM: selftests: Use a single binary for dirty/clear log test Peter Xu
2019-11-29 21:35 ` [PATCH RFC 11/15] KVM: selftests: Introduce after_vcpu_run hook for dirty " Peter Xu
2019-11-29 21:35 ` [PATCH RFC 12/15] KVM: selftests: Add dirty ring buffer test Peter Xu
2019-11-29 21:35 ` [PATCH RFC 13/15] KVM: selftests: Let dirty_log_test async for dirty ring test Peter Xu
2019-11-29 21:35 ` [PATCH RFC 14/15] KVM: selftests: Add "-c" parameter to dirty log test Peter Xu
2019-11-29 21:35 ` [PATCH RFC 15/15] KVM: selftests: Test dirty ring waitqueue Peter Xu
2019-11-30 8:29 ` [PATCH RFC 00/15] KVM: Dirty ring interface Paolo Bonzini
2019-12-02 2:13 ` Peter Xu
2019-12-03 13:59 ` Paolo Bonzini
2019-12-05 19:30 ` Peter Xu
2019-12-05 19:59 ` Paolo Bonzini
2019-12-05 20:52 ` Peter Xu
2019-12-02 20:21 ` Sean Christopherson
2019-12-02 20:43 ` Peter Xu
2019-12-04 10:39 ` Jason Wang
2019-12-04 19:33 ` Peter Xu
2019-12-05 6:49 ` Jason Wang
2019-12-11 13:41 ` Christophe de Dinechin
2019-12-11 14:16 ` Paolo Bonzini
2019-12-11 17:15 ` Peter Xu
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