Re: [RFC PATCH 0/3] KVM: x86: introduce pv feature lazy tscdeadline

[Wang Jianchao]

On 2023.07.07 14:38, Wang Jianchao wrote:
> 
> Hi
> 
> This patchset attemps to introduce a new pv feature, lazy tscdeadline.
> Everytime guest write msr of MSR_IA32_TSC_DEADLINE, a vm-exit occurs
> and host side handle it. However, a lot of the vm-exit is unnecessary
> because the timer is often over-written before it expires. 
> 
> v : write to msr of tsc deadline
> | : timer armed by tsc deadline
> 
>          v v v v v        | | | | |
> --------------------------------------->  Time
> 
> The timer armed by msr write is over-written before expires and the
> vm-exit caused by it are wasted. The lazy tscdeadline works as following,
> 
>          v v v v v        |       |
> --------------------------------------->  Time
>                           '- arm -'
> 
> The 1st timer is responsible for arming the next timer. When the armed
> timer is expired, it will check pending and arm a new timer.
> 
> In the netperf test with TCP_RR on loopback, this lazy_tscdeadline can
> reduce vm-exit obviously.
> 
>                          Close               Open
> --------------------------------------------------------
> VM-Exit
>              sum         12617503            5815737
>             intr      0% 37023            0% 33002
>            cpuid      0% 1                0% 0
>             halt     19% 2503932         47% 2780683
>        msr-write     79% 10046340        51% 2966824
>            pause      0% 90               0% 84
>    ept-violation      0% 584              0% 336
>    ept-misconfig      0% 0                0% 2
> preemption-timer      0% 29518            0% 34800
> -------------------------------------------------------
> MSR-Write
>             sum          10046455            2966864
>         apic-icr     25% 2533498         93% 2781235
>     tsc-deadline     74% 7512945          6% 185629
> 

There has not been any patches on qemu side, I open this feature with a
debug patch as attachment. It is to make the test more convenient which
can open the feature w/o involving the qemu. If you want to test this
feature, it may help.

echo 1 > /proc/sys/kernel/apic_lazy_tsc_deadline

Since it is just for testing, the serializing is not so exact.
Please use it w/o any running guests ;)

Thanks
Jianchao

> This patchset is made and tested on 6.4.0, includes 3 patches,
> 
> The 1st one adds necessary data structures for this feature
> The 2nd one adds the specific msr operations between guest and host
> The 3rd one are the one make this feature works.
> 
> Any comment is welcome.
> 
> Thanks
> Jianchao
> 
> Wang Jianchao (3)
> 	KVM: x86: add msr register and data structure for lazy tscdeadline
> 	KVM: x86: exchange info about lazy_tscdeadline with msr
> 	KVM: X86: add lazy tscdeadline support to reduce vm-exit of msr-write
> 
> 
>  arch/x86/include/asm/kvm_host.h      |  10 ++++++++
>  arch/x86/include/uapi/asm/kvm_para.h |   9 +++++++
>  arch/x86/kernel/apic/apic.c          |  47 ++++++++++++++++++++++++++++++++++-
>  arch/x86/kernel/kvm.c                |  13 ++++++++++
>  arch/x86/kvm/cpuid.c                 |   1 +
>  arch/x86/kvm/lapic.c                 | 128 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++------
>  arch/x86/kvm/lapic.h                 |   4 +++
>  arch/x86/kvm/x86.c                   |  26 ++++++++++++++++++++
>  8 files changed, 229 insertions(+), 9 deletions(-)
>