KVM Archive on lore.kernel.org
 help / color / Atom feed
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
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


  reply index

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

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20191216044619-mutt-send-email-mst@kernel.org \
    --to=mst@redhat.com \
    --cc=dgilbert@redhat.com \
    --cc=kvm@vger.kernel.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=pbonzini@redhat.com \
    --cc=peterx@redhat.com \
    --cc=sean.j.christopherson@intel.com \
    --cc=vkuznets@redhat.com \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

KVM Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/kvm/0 kvm/git/0.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 kvm kvm/ https://lore.kernel.org/kvm \
		kvm@vger.kernel.org
	public-inbox-index kvm

Example config snippet for mirrors

Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.kernel.vger.kvm


AGPL code for this site: git clone https://public-inbox.org/public-inbox.git