Re: [PATCH RFC 00/15] KVM: Dirty ring interface

Paolo Bonzini
2019-11-30T08:30:05Z

Hi Peter,

thanks for the RFC! Just a couple comments before I look at the series
(for which I don't expect many surprises).

On 29/11/19 22:34, Peter Xu wrote:
> 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.
> */
> r = kvm_emulate_instruction(vcpu, 0);
> r = r >= 0 ? 0 : r;
> goto out;
> }

This is not needed, it will just be a false negative (dirty page that
actually isn't dirty). The dirty bit will be cleared when userspace
resets the ring buffer; then the instruction will be executed again and
mark the page dirty again. Since ring full is not a common condition,
it's not a big deal.

> 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).

One question about this testcase: why does the task get into
uninterruptible wait?

Paolo

>
> 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
>