Date:   Wed, 11 Dec 2019 15:16:30 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Christophe de Dinechin <dinechin@...hat.com>,
        Peter Xu <peterx@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>
Subject: Re: [PATCH RFC 00/15] KVM: Dirty ring interface

On 11/12/19 14:41, Christophe de Dinechin wrote:
> 
> Peter Xu writes:
> 
>> Branch is here: https://github.com/xzpeter/linux/tree/kvm-dirty-ring
>>
>> Overview
>> ============
>>
>> This is a continued work from Lei Cao <lei.cao@...atus.com> and Paolo
>> on the KVM dirty ring interface.  To make it simple, I'll still start
>> with version 1 as RFC.
>>
>> The new dirty ring interface is another way to collect dirty pages for
>> the virtual machine, but it is different from the existing dirty
>> logging interface in a few ways, majorly:
>>
>>   - Data format: The dirty data was in a ring format rather than a
>>     bitmap format, so the size of data to sync for dirty logging does
>>     not depend on the size of guest memory any more, but speed of
>>     dirtying.  Also, the dirty ring is per-vcpu (currently plus
>>     another per-vm ring, so total ring number is N+1), while the dirty
>>     bitmap is per-vm.
> 
> I like Sean's suggestion to fetch rings when dirtying. That could reduce
> the number of dirty rings to examine.

What do you mean by "fetch rings"?

> Also, as is, this means that the same gfn may be present in multiple
> rings, right?

I think the actual marking of a page as dirty is protected by a spinlock
but I will defer to Peter on this.

Paolo

>>
>>   - Data copy: The sync of dirty pages does not need data copy any more,
>>     but instead the ring is shared between the userspace and kernel by
>>     page sharings (mmap() on either the vm fd or vcpu fd)
>>
>>   - Interface: Instead of using the old KVM_GET_DIRTY_LOG,
>>     KVM_CLEAR_DIRTY_LOG interfaces, the new ring uses a new interface
>>     called KVM_RESET_DIRTY_RINGS when we want to reset the collected
>>     dirty pages to protected mode again (works like
>>     KVM_CLEAR_DIRTY_LOG, but ring based)
>>
>> And more.
>>
>> I would appreciate if the reviewers can start with patch "KVM:
>> Implement ring-based dirty memory tracking", especially the document
>> update part for the big picture.  Then I'll avoid copying into most of
>> them into cover letter again.
>>
>> I marked this series as RFC because I'm at least uncertain on this
>> change of vcpu_enter_guest():
>>
>>         if (kvm_check_request(KVM_REQ_DIRTY_RING_FULL, vcpu)) {
>>                 vcpu->run->exit_reason = KVM_EXIT_DIRTY_RING_FULL;
>>                 /*
>>                         * If this is requested, it means that we've
>>                         * marked the dirty bit in the dirty ring BUT
>>                         * we've not written the date.  Do it now.
> 
> not written the "data" ?
> 
>>                         */
>>                 r = kvm_emulate_instruction(vcpu, 0);
>>                 r = r >= 0 ? 0 : r;
>>                 goto out;
>>         }
>>
>> I did a kvm_emulate_instruction() when dirty ring reaches softlimit
>> and want to exit to userspace, however I'm not really sure whether
>> there could have any side effect.  I'd appreciate any comment of
>> above, or anything else.
>>
>> Tests
>> ===========
>>
>> I wanted to continue work on the QEMU part, but after I noticed that
>> the interface might still prone to change, I posted this series first.
>> However to make sure it's at least working, I've provided unit tests
>> together with the series.  The unit tests should be able to test the
>> series in at least three major paths:
>>
>>   (1) ./dirty_log_test -M dirty-ring
>>
>>       This tests async ring operations: this should be the major work
>>       mode for the dirty ring interface, say, when the kernel is
>>       queuing more data, the userspace is collecting too.  Ring can
>>       hardly reaches full when working like this, because in most
>>       cases the collection could be fast.
>>
>>   (2) ./dirty_log_test -M dirty-ring -c 1024
>>
>>       This set the ring size to be very small so that ring soft-full
>>       always triggers (soft-full is a soft limit of the ring state,
>>       when the dirty ring reaches the soft limit it'll do a userspace
>>       exit and let the userspace to collect the data).
>>
>>   (3) ./dirty_log_test -M dirty-ring-wait-queue
>>
>>       This sololy test the extreme case where ring is full.  When the
>>       ring is completely full, the thread (no matter vcpu or not) will
>>       be put onto a per-vm waitqueue, and KVM_RESET_DIRTY_RINGS will
>>       wake the threads up (assuming until which the ring will not be
>>       full any more).
> 
> Am I correct assuming that guest memory can be dirtied by DMA operations?
> Should
> 
> Not being that familiar with the current implementation of dirty page
> tracking, I wonder who marks the pages dirty in that case, and when?
> If the VM ring is used for I/O threads, isn't it possible that a large
> DMA could dirty a sufficiently large number of GFNs to overflow the
> associated ring? Does this case need a separate way to queue the
> dirtying I/O thread?
> 
>>
>> Thanks,
>>
>> Cao, Lei (2):
>>   KVM: Add kvm/vcpu argument to mark_dirty_page_in_slot
>>   KVM: X86: Implement ring-based dirty memory tracking
>>
>> Paolo Bonzini (1):
>>   KVM: Move running VCPU from ARM to common code
>>
>> Peter Xu (12):
>>   KVM: Add build-time error check on kvm_run size
>>   KVM: Implement ring-based dirty memory tracking
>>   KVM: Make dirty ring exclusive to dirty bitmap log
>>   KVM: Introduce dirty ring wait queue
>>   KVM: selftests: Always clear dirty bitmap after iteration
>>   KVM: selftests: Sync uapi/linux/kvm.h to tools/
>>   KVM: selftests: Use a single binary for dirty/clear log test
>>   KVM: selftests: Introduce after_vcpu_run hook for dirty log test
>>   KVM: selftests: Add dirty ring buffer test
>>   KVM: selftests: Let dirty_log_test async for dirty ring test
>>   KVM: selftests: Add "-c" parameter to dirty log test
>>   KVM: selftests: Test dirty ring waitqueue
>>
>>  Documentation/virt/kvm/api.txt                | 116 +++++
>>  arch/arm/include/asm/kvm_host.h               |   2 -
>>  arch/arm64/include/asm/kvm_host.h             |   2 -
>>  arch/x86/include/asm/kvm_host.h               |   5 +
>>  arch/x86/include/uapi/asm/kvm.h               |   1 +
>>  arch/x86/kvm/Makefile                         |   3 +-
>>  arch/x86/kvm/mmu/mmu.c                        |   6 +
>>  arch/x86/kvm/vmx/vmx.c                        |   7 +
>>  arch/x86/kvm/x86.c                            |  12 +
>>  include/linux/kvm_dirty_ring.h                |  67 +++
>>  include/linux/kvm_host.h                      |  37 ++
>>  include/linux/kvm_types.h                     |   1 +
>>  include/uapi/linux/kvm.h                      |  36 ++
>>  tools/include/uapi/linux/kvm.h                |  47 ++
>>  tools/testing/selftests/kvm/Makefile          |   2 -
>>  .../selftests/kvm/clear_dirty_log_test.c      |   2 -
>>  tools/testing/selftests/kvm/dirty_log_test.c  | 452 ++++++++++++++++--
>>  .../testing/selftests/kvm/include/kvm_util.h  |   6 +
>>  tools/testing/selftests/kvm/lib/kvm_util.c    | 103 ++++
>>  .../selftests/kvm/lib/kvm_util_internal.h     |   5 +
>>  virt/kvm/arm/arm.c                            |  29 --
>>  virt/kvm/arm/perf.c                           |   6 +-
>>  virt/kvm/arm/vgic/vgic-mmio.c                 |  15 +-
>>  virt/kvm/dirty_ring.c                         | 156 ++++++
>>  virt/kvm/kvm_main.c                           | 315 +++++++++++-
>>  25 files changed, 1329 insertions(+), 104 deletions(-)
>>  create mode 100644 include/linux/kvm_dirty_ring.h
>>  delete mode 100644 tools/testing/selftests/kvm/clear_dirty_log_test.c
>>  create mode 100644 virt/kvm/dirty_ring.c
> 
> 
> --
> Cheers,
> Christophe de Dinechin (IRC c3d)
> 

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