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

Konrad Rzeszutek Wilk
2021-02-08T19:58:14Z

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
>