Background snapshots

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