[RFC PATCH 0/4] Add support for ARMv8.6 TWED feature

[Jingyi Wang]

Hi all，

Sorry for the delay. I have been testing the TWED feature performance
lately. We select unixbench as the benchmark for some items of it is 
lock-intensive(fstime/fsbuffer/fsdisk). We run unixbench on a 4-VCPU
VM, and bind every two VCPUs on one PCPU. Fixed TWED value is used and 
here is the result.

      twed_value   | fstime        | fsbuffer   | fsdisk
     --------------+---------------+------------+------------
      disable      | 16.0          | 14.1       | 18.0
      0            | 16.3          | 13.5       | 17.2
      1            | 17.5          | 14.7       | 17.4
      2            | 17.3          | 15.3       | 18.0
      3            | 17.7          | 15.2       | 18.9
      4            | 17.9          | 14.3       | 18.2
      5            | 17.2          | 14.1       | 19.0
      6            | 5.8           | 4.2        | 5.7
      7            | 6.2           | 5.6        | 12.8

Note:
fstime: File Copy 1024 bufsize 2000 maxblocks
fsbuffer: File Copy 256 bufsize 500 maxblocks
fsdisk: File Copy 4096 bufsize 8000 maxblocks
The index of unixbench, higher is better.

It is shown that, compared to the circumstance that TWED is disabled,
lock-intensive testing items have better performance if an appropriate
TWED value is set(up to 5.6%~11.9%). Meanwhile, the complete unixbench
test is run to prove that other testing items are not sensitive to this
parameter.

Thanks
Jingyi

On 9/29/2020 5:17 PM, Jingyi Wang wrote:
> TWE Delay is an optional feature in ARMv8.6 Extentions. There is a
> performance benefit in waiting for a period of time for an event to
> arrive before taking the trap as it is common that event will arrive
> “quite soon” after executing the WFE instruction.
> 
> This series adds support for TWED feature and implements TWE delay
> value dynamic adjustment.
> 
> Thanks for Shameer's advice on this series. The function of this patch
> has been tested on TWED supported hardware and the performance of it is
> still on test, any advice will be welcomed.
> 
> Jingyi Wang (2):
>    KVM: arm64: Make use of TWED feature
>    KVM: arm64: Use dynamic TWE Delay value
> 
> Zengruan Ye (2):
>    arm64: cpufeature: TWED support detection
>    KVM: arm64: Add trace for TWED update
> 
>   arch/arm64/Kconfig                   | 10 +++++
>   arch/arm64/include/asm/cpucaps.h     |  3 +-
>   arch/arm64/include/asm/kvm_arm.h     |  5 +++
>   arch/arm64/include/asm/kvm_emulate.h | 38 ++++++++++++++++++
>   arch/arm64/include/asm/kvm_host.h    | 19 ++++++++-
>   arch/arm64/include/asm/virt.h        |  8 ++++
>   arch/arm64/kernel/cpufeature.c       | 12 ++++++
>   arch/arm64/kvm/arm.c                 | 58 ++++++++++++++++++++++++++++
>   arch/arm64/kvm/handle_exit.c         |  2 +
>   arch/arm64/kvm/trace_arm.h           | 21 ++++++++++
>   10 files changed, 174 insertions(+), 2 deletions(-)
>