linux-kernel - Re: [RESEND RFC: timer passthrough 0/9] Support timer passthrough for VM

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Date:   Mon, 8 Feb 2021 13:13:59 -0500
From:   Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:     Zhimin Feng <fengzhimin@...edance.com>
Cc:     x86@...nel.org, kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        pbonzini@...hat.com, seanjc@...gle.com, vkuznets@...hat.com,
        wanpengli@...cent.com, jmattson@...gle.com, joro@...tes.org,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, hpa@...or.com,
        fweisbec@...il.com, zhouyibo@...edance.com,
        zhanghaozhong@...edance.com
Subject: Re: [RESEND RFC: timer passthrough 0/9] Support timer passthrough
 for VM

On Fri, Feb 05, 2021 at 06:03:08PM +0800, Zhimin Feng wrote:
> The main motivation for this patch is to improve the performance of VM.
> This patch series introduces how to enable the timer passthrough in
> non-root mode.

Nice! Those are impressive numbers!

> 
> The main idea is to offload the host timer to the preemtion timer in
> non-root mode. Through doing this, guest can write tscdeadline msr directly
> in non-root mode and host timer isn't lost. If CPU is in root mode,
> guest timer is switched to software timer.

I am sorry - but I am having a hard time understanding the sentence
above so let me ask some specific questions.

- How do you protect against the guest DoS-ing the host and mucking with
  the host timer?

- As in can you explain how the host can still continue scheduling it's
  own quanta?

And one more - what happens with Live Migration? I would assume that
becomes a no-go anymore unless you swap in the guest timer back in? So
we end up emulating the MSR again?

Thanks!

> 
> Testing on Intel(R) Xeon(R) Platinum 8260 server.
> 
> The guest OS is Debian(kernel: 4.19.28). The specific configuration is
>  is as follows: 8 cpu, 16GB memory, guest idle=poll
> memcached in guest(memcached -d -t 8 -u root)
> 
> I use the memtier_benchmark tool to test performance
> (memtier_benchmark -P memcache_text -s guest_ip -c 16 -t 32
>  --key-maximum=10000000000 --random-data --data-size-range=64-128 -p 11211
>  --generate-keys --ratio 5:1 --test-time=500)
> 
> Total Ops can be improved 25% and Avg.Latency can be improved 20% when
> the timer-passthrough is enabled.
> 
> =============================================================
>                | Enable timer-passth | Disable timer-passth |
> =============================================================
> Totals Ops/sec |    514869.67        |     411766.67        |
> -------------------------------------------------------------
> Avg.Latency    |    0.99483          |     1.24294          |
> =============================================================
> 
> 
> Zhimin Feng (9):
>   KVM: vmx: hook set_next_event for getting the host tscd
>   KVM: vmx: enable host lapic timer offload preemtion timer
>   KVM: vmx: enable passthrough timer to guest
>   KVM: vmx: enable passth timer switch to sw timer
>   KVM: vmx: use tsc_adjust to enable tsc_offset timer passthrough
>   KVM: vmx: check enable_timer_passth strictly
>   KVM: vmx: save the initial value of host tscd
>   KVM: vmx: Dynamically open or close the timer-passthrough for pre-vm
>   KVM: vmx: query the state of timer-passth for vm
> 
>  arch/x86/include/asm/kvm_host.h |  27 ++++
>  arch/x86/kvm/lapic.c            |   1 +
>  arch/x86/kvm/vmx/vmx.c          | 331 +++++++++++++++++++++++++++++++++++++++-
>  arch/x86/kvm/x86.c              |  26 +++-
>  include/linux/kvm_host.h        |   1 +
>  include/uapi/linux/kvm.h        |   3 +
>  kernel/time/tick-common.c       |   1 +
>  tools/include/uapi/linux/kvm.h  |   3 +
>  virt/kvm/kvm_main.c             |   1 +
>  9 files changed, 389 insertions(+), 5 deletions(-)
> 
> -- 
> 2.11.0
>