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

From: Paolo Bonzini
Date: Mon Nov 26 2018 - 11:54:21 EST

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
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 and ARM).

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 | 78 ++++++++++-
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 | 147 ++++++++++++++++++---
14 files changed, 345 insertions(+), 65 deletions(-)
create mode 100644 tools/testing/selftests/kvm/clear_dirty_log_test.c