[v2,0/3] kvm: split retrieval and clearing of dirty log

Message

Paolo Bonzini Nov. 28, 2018, 11:42 a.m. UTC

There are two problems with KVM_GET_DIRTY_LOG.  First, and less important,
it can take kvm->mmu_lock for an extended period of time.  Second, its user
can actually see many false positives in some cases.  The latter is due
to a benign race like this:

  1. KVM_GET_DIRTY_LOG returns a set of dirty pages and write protects
     them.
  2. The guest modifies the pages, causing them to be marked ditry.
  3. Userspace actually copies the pages.
  4. KVM_GET_DIRTY_LOG returns those pages as dirty again, even though
     they were not written to since (3).

This is especially a problem for large guests, where the time between
(1) and (3) can be substantial.  This patch introduces a new
capability which, when enabled, makes KVM_GET_DIRTY_LOG not
write-protect the pages it returns.  Instead, userspace has to
explicitly clear the dirty log bits just before using the content
of the page.  The new KVM_CLEAR_DIRTY_LOG ioctl can operate on a
64-page granularity rather than requiring to sync a full memslot.
This way the mmu_lock is taken for small amounts of time, and
only a small amount of time will pass between write protection
of pages and the sending of their content.

This is entirely implemented in generic code, but only users of
kvm_get_dirty_log_protect get the support (that is x86_64, ARM and MIPS).

There are no code changes from v1, only documentation and comments.

v1->v2: fix documentation and comments to cover PML case [Junaid]
	fix parameter direction [Junaid]
	remark on userspace setting bits beyond the end of the memslot [Junaid]

Paolo Bonzini (3):
  kvm: make KVM_CAP_ENABLE_CAP_VM architecture agnostic
  kvm: rename last argument to kvm_get_dirty_log_protect
  kvm: introduce manual dirty log reprotect

 Documentation/virtual/kvm/api.txt                  |  80 ++++++++++-
 arch/mips/kvm/mips.c                               |  29 +++-
 arch/powerpc/kvm/powerpc.c                         |  14 +-
 arch/s390/kvm/kvm-s390.c                           |  11 +-
 arch/x86/kvm/x86.c                                 |  47 ++++--
 include/linux/kvm_host.h                           |   9 +-
 include/uapi/linux/kvm.h                           |  15 ++
 tools/testing/selftests/kvm/Makefile               |   2 +
 tools/testing/selftests/kvm/clear_dirty_log_test.c |   2 +
 tools/testing/selftests/kvm/dirty_log_test.c       |  19 +++
 tools/testing/selftests/kvm/include/kvm_util.h     |   2 +
 tools/testing/selftests/kvm/lib/kvm_util.c         |  13 ++
 virt/kvm/arm/arm.c                                 |  22 ++-
 virt/kvm/kvm_main.c                                | 159 ++++++++++++++++++---
 14 files changed, 358 insertions(+), 66 deletions(-)
 create mode 100644 tools/testing/selftests/kvm/clear_dirty_log_test.c