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Subject[PATCH RFCv2 0/9] kvm/arm64: Support Async Page Fault
Date
There are two stages of page faults and the stage one page fault is
handled by guest itself. The guest is trapped to host when the page
fault is caused by stage 2 page table, for example missing. The guest
is suspended until the requested page is populated. There might be
IO activities involved for host to populate the requested page. For
instance, the requested page has been swapped out previously. In this
case, the guest (vCPU) has to suspend for a few of milliseconds, which
depends on the swapping media, regardless of the overall system load.

The series adds asychornous page fault to improve the situation. A
signal (PAGE_NOT_PRESENT) is sent from host to the guest if the requested
page isn't absent immediately. In the mean while, a worker is started
to populate the requested page in background. Guest either picks another
available process to run or puts current (faulting) process to power
saving mode when receiving the (PAGE_NOT_PRESENT) signal. After the
requested page is populated by the worker, another signal (PAGE_READY)
is sent from host to guest. Guest wakes up the (faulting) process when
receiving the (PAGE_READY) signal.

The signals are conveyed through control block. The control block physical
address is passed from guest to host through dedicated KVM vendor specific
hypercall. The control block is visible and accessible by host and guest
in the mean while. The hypercall is also used to enable, disable, configure
the functionality. Notifications, by injected abort data exception, are
fired when there are pending signals. The exception handler will be invoked
in guest kernel.

Testing
=======
The tests are carried on the following machine. A guest with single vCPU
and 4GB memory is started. Also, the QEMU process is put into memory cgroup
(v1) whose memory limit is set to 2GB. In the guest, there are two threads,
which are memory bound and CPU bound separately. The memory bound thread
allocates all available memory, accesses and them free them. The CPU bound
thread simply executes block of "nop". The test is carried out for 5 time
continuously and the average number (per minute) of executed blocks in the
CPU bound thread is taken as indicator of improvement.

Vendor: GIGABYTE CPU: 224 x Cavium ThunderX2(R) CPU CN9975 v2.2 @ 2.0GHz
Memory: 32GB Disk: Fusion-MPT SAS-3 (PCIe3.0 x8)

Without-APF: 7029030180/minute = avg(7559625120 5962155840 7823208540
7629633480 6170527920)
With-APF: 8286827472/minute = avg(8464584540 8177073360 8262723180
8095084020 8434672260)
Outcome: +17.8%

Another test case is to measure the time consumed by the application, but
with the CPU-bound thread disabled.

Without-APF: 40.3s = avg(40.6 39.3 39.2 41.6 41.2)
With-APF: 40.8s = avg(40.6 41.1 40.9 41.0 40.7)
Outcome: +1.2%

I also have some code in the host to capture the number of async page faults,
time used to do swapin and its maximal/minimal values when async page fault
is enabled. During the test, the CPU-bound thread is disabled. There is about
30% of the time used to do swapin.

Number of async page fault: 7555 times
Total time used by application: 42.2 seconds
Total time used by swapin: 12.7 seconds (30%)
Minimal swapin time: 36.2 us
Maximal swapin time: 55.7 ms

Changelog
=========
RFCv1 -> RFCv2
* Rebase to 5.7.rc3
* Performance data (Marc Zyngier)
* Replace IMPDEF system register with KVM vendor specific hypercall (Mark Rutland)
* Based on Will's KVM vendor hypercall probe mechanism (Will Deacon)
* Don't use IMPDEF DFSC (0x43). Async page fault reason is conveyed
by the control block (Mark Rutland)
* Delayed wakeup mechanism in guest kernel (Gavin Shan)
* Stability improvement in the guest kernel: delayed wakeup mechanism,
external abort disallowed region, lazily clear async page fault,
disabled interrupt on acquiring the head's lock and so on (Gavin Shan)
* Stability improvement in the host kernel: serialized async page
faults etc. (Gavin Shan)
* Performance improvement in guest kernel: percpu sleeper head (Gavin Shan)

Gavin Shan (7):
kvm/arm64: Rename kvm_vcpu_get_hsr() to kvm_vcpu_get_esr()
kvm/arm64: Detach ESR operator from vCPU struct
kvm/arm64: Replace hsr with esr
kvm/arm64: Export kvm_handle_user_mem_abort() with prefault mode
kvm/arm64: Support async page fault
kernel/sched: Add cpu_rq_is_locked()
arm64: Support async page fault

Will Deacon (2):
arm64: Probe for the presence of KVM hypervisor services during boot
arm/arm64: KVM: Advertise KVM UID to guests via SMCCC

arch/arm64/Kconfig | 11 +
arch/arm64/include/asm/exception.h | 3 +
arch/arm64/include/asm/hypervisor.h | 11 +
arch/arm64/include/asm/kvm_emulate.h | 83 +++--
arch/arm64/include/asm/kvm_host.h | 47 +++
arch/arm64/include/asm/kvm_para.h | 40 +++
arch/arm64/include/uapi/asm/Kbuild | 2 -
arch/arm64/include/uapi/asm/kvm_para.h | 22 ++
arch/arm64/kernel/entry.S | 33 ++
arch/arm64/kernel/process.c | 4 +
arch/arm64/kernel/setup.c | 35 ++
arch/arm64/kvm/Kconfig | 1 +
arch/arm64/kvm/Makefile | 2 +
arch/arm64/kvm/handle_exit.c | 48 +--
arch/arm64/kvm/hyp/switch.c | 33 +-
arch/arm64/kvm/hyp/vgic-v2-cpuif-proxy.c | 7 +-
arch/arm64/kvm/inject_fault.c | 4 +-
arch/arm64/kvm/sys_regs.c | 38 +-
arch/arm64/mm/fault.c | 434 +++++++++++++++++++++++
include/linux/arm-smccc.h | 32 ++
include/linux/sched.h | 1 +
kernel/sched/core.c | 8 +
virt/kvm/arm/arm.c | 40 ++-
virt/kvm/arm/async_pf.c | 335 +++++++++++++++++
virt/kvm/arm/hyp/aarch32.c | 4 +-
virt/kvm/arm/hyp/vgic-v3-sr.c | 7 +-
virt/kvm/arm/hypercalls.c | 37 +-
virt/kvm/arm/mmio.c | 27 +-
virt/kvm/arm/mmu.c | 69 +++-
29 files changed, 1264 insertions(+), 154 deletions(-)
create mode 100644 arch/arm64/include/asm/kvm_para.h
create mode 100644 arch/arm64/include/uapi/asm/kvm_para.h
create mode 100644 virt/kvm/arm/async_pf.c

--
2.23.0

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