Re: [Qemu-devel] [PATCH v0 0/7] Background snapshots

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Denis Plotnikov (dplotnikov@virtuozzo.com) wrote:
> The patch set adds the ability to make external snapshots while VM is running.

cc'ing in Andrea since this uses sigsegv's to avoid userfault-wp that
isn't there yet.

Hi Denis,
  How robust are you finding this SEGV based trick; for example what
about things like the kernel walking vhost queues or similar kernel
nasties?

Dave

> The workflow to make a snapshot is the following:
> 1. Pause the vm
> 2. Make a snapshot of block devices using the scheme of your choice
> 3. Turn on background-snapshot migration capability
> 4. Start the migration using the destination (migration stream) of your choice.
>    The migration will resume the vm execution by itself
>    when it has the devices' states saved  and is ready to start ram writing
>    to the migration stream.
> 5. Listen to the migration finish event
> 
> The feature relies on KVM unapplied ability to report the faulting address.
> Please find the KVM patch snippet to make the patchset work below:
> 
> +++ b/arch/x86/kvm/vmx.c
> @@ -XXXX,X +XXXX,XX @@ static int handle_ept_violation(struct kvm_vcpu *vcpu)
>  
>         vcpu->arch.exit_qualification = exit_qualification;
>  
> -       return kvm_mmu_page_fault(vcpu, gpa, error_code, NULL, 0);
> +       r = kvm_mmu_page_fault(vcpu, gpa, error_code, NULL, 0);
> +        if (r == -EFAULT) {
> +               unsigned long hva = kvm_vcpu_gfn_to_hva(vcpu, gpa >> PAGE_SHIFT);
> +
> +               vcpu->run->exit_reason = KVM_EXIT_FAIL_MEM_ACCESS;
> +               vcpu->run->hw.hardware_exit_reason = EXIT_REASON_EPT_VIOLATION;
> +               vcpu->run->fail_mem_access.hva = hva | (gpa & (PAGE_SIZE-1));
> +               r = 0;
> +
> +       }
> +       return r;
> 
> The patch to KVM can be sent if the patch set approved
> 
> Denis Plotnikov (7):
>   migration: add background snapshot capability
>   bitops: add some atomic versions of bitmap operations
>   threads: add infrastructure to process sigsegv
>   migration: add background snapshot infrastructure
>   kvm: add failed memeory access exit reason
>   kvm: add vCPU failed memeory access processing
>   migration: add background snapshotting
> 
>  include/exec/ram_addr.h   |   7 +
>  include/exec/ramlist.h    |   4 +-
>  include/qemu/bitops.h     |  24 +++
>  include/qemu/thread.h     |   5 +
>  linux-headers/linux/kvm.h |   5 +
>  migration/migration.c     | 141 +++++++++++++++-
>  migration/migration.h     |   1 +
>  migration/ram.c           | 333 ++++++++++++++++++++++++++++++++++++--
>  migration/ram.h           |  11 +-
>  migration/savevm.c        |  91 ++++++-----
>  migration/savevm.h        |   2 +
>  qapi/migration.json       |   6 +-
>  target/i386/kvm.c         |  18 +++
>  util/qemu-thread-posix.c  |  50 ++++++
>  14 files changed, 635 insertions(+), 63 deletions(-)
> 
> -- 
> 2.17.0
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK