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From: Peter Xu <peterx@redhat.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Jason Wang <jasowang@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>,
	"Michael S. Tsirkin" <mst@redhat.com>
Subject: Re: [PATCH RFC 04/15] KVM: Implement ring-based dirty memory tracking
Date: Wed, 4 Dec 2019 14:52:30 -0500
Message-ID: <20191204195230.GF19939@xz-x1> (raw)
In-Reply-To: <a3e83e6b-4bfa-3a6b-4b43-5dd451e03254@redhat.com>

On Wed, Dec 04, 2019 at 12:04:53PM +0100, Paolo Bonzini wrote:
> On 04/12/19 11:38, Jason Wang wrote:
> >>
> >> +    entry = &ring->dirty_gfns[ring->dirty_index & (ring->size - 1)];
> >> +    entry->slot = slot;
> >> +    entry->offset = offset;
> > 
> > 
> > Haven't gone through the whole series, sorry if it was a silly question
> > but I wonder things like this will suffer from similar issue on
> > virtually tagged archs as mentioned in [1].
> 
> There is no new infrastructure to track the dirty pages---it's just a
> different way to pass them to userspace.
> 
> > Is this better to allocate the ring from userspace and set to KVM
> > instead? Then we can use copy_to/from_user() friends (a little bit slow
> > on recent CPUs).
> 
> Yeah, I don't think that would be better than mmap.

Yeah I agree, because I didn't see how copy_to/from_user() helped to
do icache/dcache flushings...

Some context here: Jason raised this question offlist first on whether
we should also need these flush_dcache_cache() helpers for operations
like kvm dirty ring accesses.  I feel like it should, however I've got
two other questions, on:

  - if we need to do flush_dcache_page() on kernel modified pages
    (assuming the same page has mapped to userspace), then why don't
    we need flush_cache_page() too on the page, where
    flush_cache_page() is defined not-a-nop on those archs?

  - assuming an arch has not-a-nop impl for flush_[d]cache_page(),
    would atomic operations like cmpxchg really work for them
    (assuming that ISAs like cmpxchg should depend on cache
    consistency).

Sorry I think these are for sure a bit out of topic for kvm dirty ring
patchset, but since we're at it, I'm raising the questions up in case
there're answers..

Thanks,

-- 
Peter Xu


  reply index

Thread overview: 55+ 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
2019-11-29 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
2019-12-03 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 [this message]
2019-12-05  6:51         ` Jason Wang
2019-12-05 12:08           ` Peter Xu
2019-12-05 13:12             ` Jason Wang
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

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