[PATCH v7 00/14] KVM: Dirty ring interface

Peter Xu
2020-03-18T16:37:50Z

KVM branch:
https://github.com/xzpeter/linux/tree/kvm-dirty-ring

QEMU branch for testing:
https://github.com/xzpeter/qemu/tree/kvm-dirty-ring

v7
- rebase to kvm/queue
- make as_id an u16 [Sean]
- init_rmode_tss: rename idx to i, remove data init to zero [Sean]
- update doc for __x86_set_memory_region on ret value after rebase [Sean]
- fix sparse warnings for __x86_set_memory_region [syzbot]
- test cases [Drew]
- use print_skip()
- dump log modes without '\b'
- support "all" log mode
- exit(s/-1/1/)
- assert gfns mmap in vcpu_map_dirty_ring
- make vcpu_thread/current_vm static

For previous versions, please refer to:

V1: https://lore.kernel.org/kvm/20191129213505.18472-1-peterx@redhat.com
V2: https://lore.kernel.org/kvm/20191221014938.58831-1-peterx@redhat.com
V3: https://lore.kernel.org/kvm/20200109145729.32898-1-peterx@redhat.com
V4: https://lore.kernel.org/kvm/20200205025105.367213-1-peterx@redhat.com
V5: https://lore.kernel.org/kvm/20200304174947.69595-1-peterx@redhat.com
V6: https://lore.kernel.org/kvm/20200309214424.330363-1-peterx@redhat.com

Overview
============

This is a continued work from Lei Cao <lei.cao@stratus.com> and Paolo
Bonzini on the KVM dirty ring interface.

The new dirty ring interface is another way to collect dirty pages for
the virtual machines. It is different from the existing dirty logging
interface in a few ways, majorly:

- Data format: The dirty data was in a ring format rather than a
bitmap format, so dirty bits to sync for dirty logging does not
depend on the size of guest memory any more, but speed of
dirtying. Also, the dirty ring is per-vcpu, while the dirty
bitmap is per-vm.

- Data copy: The sync of dirty pages does not need data copy any more,
but instead the ring is shared between the userspace and kernel by
page sharings (mmap() on vcpu fd)

- Interface: Instead of using the old KVM_GET_DIRTY_LOG,
KVM_CLEAR_DIRTY_LOG interfaces, the new ring uses the new
KVM_RESET_DIRTY_RINGS ioctl when we want to reset the collected
dirty pages to protected mode again (works like
KVM_CLEAR_DIRTY_LOG, but ring based). To collecting dirty bits,
we only need to read the ring data, no ioctl is needed.

Ring Layout
===========

KVM dirty ring is per-vcpu. Each ring is an array of kvm_dirty_gfn
defined as:

struct kvm_dirty_gfn {
__u32 flags;
__u32 slot; /* as_id | slot_id */
__u64 offset;
};

Each GFN is a state machine itself. The state is embeded in the flags
field, as defined in the uapi header:

/*
* KVM dirty GFN flags, defined as:
*
* |---------------+---------------+--------------|
* | bit 1 (reset) | bit 0 (dirty) | Status |
* |---------------+---------------+--------------|
* | 0 | 0 | Invalid GFN |
* | 0 | 1 | Dirty GFN |
* | 1 | X | GFN to reset |
* |---------------+---------------+--------------|
*
* Lifecycle of a dirty GFN goes like:
*
* dirtied collected reset
* 00 -----------> 01 -------------> 1X -------+
* ^ |
* | |
* +------------------------------------------+
*
* The userspace program is only responsible for the 01->1X state
* conversion (to collect dirty bits). Also, it must not skip any
* dirty bits so that dirty bits are always collected in sequence.
*/

Testing
=======

This series provided both the implementation of the KVM dirty ring and
the test case. Also I've implemented the QEMU counterpart that can
run with the new KVM, link can be found at the top of the cover
letter. However that's still a very initial version which is prone to
change and future optimizations.

I did some measurement with the new method with 24G guest running some
dirty workload, I don't see any speedup so far, even in some heavy
dirty load it'll be slower (e.g., when 800MB/s random dirty rate, kvm
dirty ring takes average of ~73s to complete migration while dirty
logging only needs average of ~55s). However that's understandable
because 24G guest means only 1M dirty bitmap, that's still a suitable
case for dirty logging. Meanwhile heavier workload means worst case
for dirty ring.

More tests are welcomed if there's bigger host/guest, especially on
COLO-like workload.

Please review, thanks.

Peter Xu (14):
KVM: X86: Change parameter for fast_page_fault tracepoint
KVM: Cache as_id in kvm_memory_slot
KVM: X86: Don't track dirty for KVM_SET_[TSS_ADDR|IDENTITY_MAP_ADDR]
KVM: Pass in kvm pointer into mark_page_dirty_in_slot()
KVM: X86: Implement ring-based dirty memory tracking
KVM: Make dirty ring exclusive to dirty bitmap log
KVM: Don't allocate dirty bitmap if dirty ring is enabled
KVM: selftests: Always clear dirty bitmap after iteration
KVM: selftests: Sync uapi/linux/kvm.h to tools/
KVM: selftests: Use a single binary for dirty/clear log test
KVM: selftests: Introduce after_vcpu_run hook for dirty log test
KVM: selftests: Add dirty ring buffer test
KVM: selftests: Let dirty_log_test async for dirty ring test
KVM: selftests: Add "-c" parameter to dirty log test

Documentation/virt/kvm/api.rst | 123 +++++
arch/x86/include/asm/kvm_host.h | 6 +-
arch/x86/include/uapi/asm/kvm.h | 1 +
arch/x86/kvm/Makefile | 3 +-
arch/x86/kvm/mmu/mmu.c | 10 +-
arch/x86/kvm/mmutrace.h | 9 +-
arch/x86/kvm/svm.c | 9 +-
arch/x86/kvm/vmx/vmx.c | 89 +--
arch/x86/kvm/x86.c | 50 +-
include/linux/kvm_dirty_ring.h | 103 ++++
include/linux/kvm_host.h | 19 +
include/trace/events/kvm.h | 78 +++
include/uapi/linux/kvm.h | 53 ++
tools/include/uapi/linux/kvm.h | 99 +++-
tools/testing/selftests/kvm/Makefile | 2 -
.../selftests/kvm/clear_dirty_log_test.c | 6 -
tools/testing/selftests/kvm/dirty_log_test.c | 505 ++++++++++++++++--
.../testing/selftests/kvm/include/kvm_util.h | 4 +
tools/testing/selftests/kvm/lib/kvm_util.c | 68 +++
.../selftests/kvm/lib/kvm_util_internal.h | 4 +
virt/kvm/dirty_ring.c | 195 +++++++
virt/kvm/kvm_main.c | 162 +++++-
22 files changed, 1460 insertions(+), 138 deletions(-)
create mode 100644 include/linux/kvm_dirty_ring.h
delete mode 100644 tools/testing/selftests/kvm/clear_dirty_log_test.c
create mode 100644 virt/kvm/dirty_ring.c

--
2.24.1