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

[Denis Plotnikov <dplotnikov@virtuozzo.com>]

The patch set adds the ability to make external snapshots while VM is running.

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