Re: [PATCH v3 0/6] IPI virtualization support for VM

[Zeng Guang <guang.zeng@intel.com>]

On 8/6/2021 1:03 AM, Jim Mattson wrote:
> On Thu, Aug 5, 2021 at 8:38 AM Zeng Guang <guang.zeng@intel.com> wrote:
>
> Current IPI process in guest VM will virtualize the writing to interrupt
> command register(ICR) of the local APIC which will cause VM-exit anyway
> on source vCPU. Frequent VM-exit could induce much overhead accumulated
> if running IPI intensive task.
>
> IPI virtualization as a new VT-x feature targets to eliminate VM-exits
> when issuing IPI on source vCPU. It introduces a new VM-execution
> control - "IPI virtualization"(bit4) in the tertiary processor-based
> VM-exection controls and a new data structure - "PID-pointer table
> VM-execution
>> address" and "Last PID-pointer index" referenced by the VMCS. When "IPI
>> virtualization" is enabled, processor emulateds following kind of writes
> emulates
> to APIC registers that would send IPIs, moreover without causing VM-exits.
> - Memory-mapped ICR writes
> - MSR-mapped ICR writes
> - SENDUIPI execution
>
> This patch series implement IPI virtualization support in KVM.
> implements
>> Patches 1-4 add tertiary processor-based VM-execution support
>> framework.
>>
>> Patch 5 implement interrupt dispatch support in x2APIC mode with
> implements
>> APIC-write VM exit. In previous platform, no CPU would produce
>> APIC-write VM exit with exit qulification 300H when the "virtual x2APIC
> qualification
>> mode" VM-execution control was 1.
>>
>> Patch 6 implement IPI virtualization related function including
>> feature enabling through tertiary processor-based VM-execution in
>> various scenario of VMCS configuration, PID table setup in vCPU creation
> scenarios
>> and vCPU block consideration.
>>
>> Document for IPI virtualization is now available at the latest "Intel
>> Architecture Instruction Set Extensions Programming Reference".
>>
>> Document Link:
>> https://software.intel.com/content/www/us/en/develop/download/intel-architecture-instruction-set-extensions-programming-reference.html
>>
>> We did experiment to measure average time sending IPI from source vCPU
>> to the target vCPU completing the IPI handling by kvm unittest w/ and
>> w/o IPI virtualization. When IPI virtualizatin enabled, it will reduce
> virtualization
>> 22.21% and 15.98% cycles consuming in xAPIC mode and x2APIC mode
>> respectly.
> respectively
>> KMV unittest:vmexit/ipi, 2 vCPU, AP was modified to run in idle loop
> KVM

Thanks for your all corrections.

>> instead of halt to ensure no VM exit impact on target vCPU.
> Are you going to post the kvm-init-test changes?

We modified unit test common code specific for the IPIv test purpose, 
currently no plan to post those changes to kvm-uint-tests.
>>                  Cycles of IPI
>>                  xAPIC mode              x2APIC mode
>>          test    w/o IPIv  w/ IPIv       w/o IPIv  w/ IPIv
>>          1       6106      4816          4265      3768
>>          2       6244      4656          4404      3546
>>          3       6165      4658          4233      3474
>>          4       5992      4710          4363      3430
>>          5       6083      4741          4215      3551
>>          6       6238      4904          4304      3547
>>          7       6164      4617          4263      3709
>>          8       5984      4763          4518      3779
>>          9       5931      4712          4645      3667
>>          10      5955      4530          4332      3724
>>          11      5897      4673          4283      3569
>>          12      6140      4794          4178      3598
>>          13      6183      4728          4363      3628
>>          14      5991      4994          4509      3842
>>          15      5866      4665          4520      3739
>>          16      6032      4654          4229      3701
>>          17      6050      4653          4185      3726
>>          18      6004      4792          4319      3746
>>          19      5961      4626          4196      3392
>>          20      6194      4576          4433      3760
>>
>> Average cycles  6059      4713.1        4337.85   3644.8
>> %Reduction                -22.21%                 -15.98%
>>
>> --------------------------------------
>> IPI microbenchmark:
>> (https://lore.kernel.org/kvm/20171219085010.4081-1-ynorov@caviumnetworks.com)
>>
>> 2 vCPUs, 1:1 pin vCPU to pCPU, guest VM runs with idle=poll, x2APIC mode
> For dedicated CPUs, we would just disable the MONITOR/MWAIT intercepts
> and expose MONITOR/MWAIT to the guest. We would not recommend that
> anyone use idle=poll.
OK. We thought that straightforward way to make guest keep running in 
idle without side
effect of VM exit and other factors, so used idle=poll for test.
>> Result with IPIv enabled:
>>
>> Dry-run:                         0,             272798 ns
>> Self-IPI:                  5094123,           11114037 ns
>> Normal IPI:              131697087,          173321200 ns
>> Broadcast IPI:                   0,          155649075 ns
>> Broadcast lock:                  0,          161518031 ns
>>
>> Result with IPIv disabled:
>>
>> Dry-run:                         0,             272766 ns
>> Self-IPI:                  5091788,           11123699 ns
>> Normal IPI:              145215772,          174558920 ns
>> Broadcast IPI:                   0,          175785384 ns
>> Broadcast lock:                  0,          149076195 ns
>>
>>
>> As IPIv can benefit unicast IPI to other CPU, Noraml IPI test case gain
> Normal
>> about 9.73% time saving on average out of 15 test runs when IPIv is
>> enabled.
> Can you share the CDFs?
Normal IPI statistics:
                 test            w/o IPIv  (unit:ns)        w/ IPIv 
(unit:ns)
                 1                153346049 140907046
                 2                147218648 141660618
                 3                145215772 117890672
                 4                146621682 136430470
                 5                144821472 136199421
                 6                144704378 131676928
                 7                141403224 131697087
                 8                144775766 125476250
                 9                140658192 137263330
                 10              144768626 138593127
                 11              145166679 131946752
                 12              145020451 116852889
                 13              148161353 131406280
                 14              148378655 130174353
                 15              148903652 127969674
>>                  w/o IPIv                w/ IPIv
>> Normal IPI:     145944306.6 ns          131742993.1 ns
>> %Reduction                              -9.73%
>>
>> --------------------------------------
>> hackbench:
>>
>> 8 vCPUs, guest VM free run, x2APIC mode
>> ./hackbench -p -l 100000
>>
>>                  w/o IPIv        w/ IPIv
>> Time:           91.887          74.605
>> %Reduction:                     -18.808%
>>
>> 96 vCPUs, guest VM free run, x2APIC mode
>> ./hackbench -p -l 1000000
>>
>>                  w/o IPIv        w/ IPIv
>> Time:           287.504         235.185
>> %Reduction:                     -18.198%
>>
>> --------------------------------------
>>
>> v2 -> v3:
>> 1. Misc change on tertiary execution control
>>     definition and capability setup
>> 2. Alternative to get tertiary execution
>>     control configuration
>>
>> v1 -> v2:
>> 1. Refine the IPIv enabling logic for VM.
>>     Remove ipiv_active definition per vCPU.
>>
>> Gao Chao (1):
>>    KVM: VMX: enable IPI virtualization
>>
>> Robert Hoo (4):
>>    x86/feat_ctl: Add new VMX feature, Tertiary VM-Execution control
>>    KVM: VMX: Extend BUILD_CONTROLS_SHADOW macro to support 64-bit
>>      variation
>>    KVM: VMX: Detect Tertiary VM-Execution control when setup VMCS config
>>    KVM: VMX: dump_vmcs() reports tertiary_exec_control field as well
>>
>> Zeng Guang (1):
>>    KVM: x86: Support interrupt dispatch in x2APIC mode with APIC-write VM
>>      exit
>>
>>   arch/x86/include/asm/msr-index.h   |   1 +
>>   arch/x86/include/asm/vmx.h         |  11 +++
>>   arch/x86/include/asm/vmxfeatures.h |   5 +-
>>   arch/x86/kernel/cpu/feat_ctl.c     |  11 ++-
>>   arch/x86/kvm/lapic.c               |   9 ++-
>>   arch/x86/kvm/vmx/capabilities.h    |  14 ++++
>>   arch/x86/kvm/vmx/evmcs.c           |   2 +
>>   arch/x86/kvm/vmx/evmcs.h           |   1 +
>>   arch/x86/kvm/vmx/posted_intr.c     |  22 ++++--
>>   arch/x86/kvm/vmx/vmcs.h            |   1 +
>>   arch/x86/kvm/vmx/vmx.c             | 114 +++++++++++++++++++++++++++--
>>   arch/x86/kvm/vmx/vmx.h             |  27 ++++---
>>   12 files changed, 193 insertions(+), 25 deletions(-)
>>
>> --
>> 2.25.1
>>