linux-kernel - Re: [PATCH v2 1/3] KVM: LAPIC: Make lapic timer unpinned when timer is injected by posted-interrupt

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Date:   Mon, 10 Jun 2019 19:11:10 +0200
From:   Radim Krčmář <rkrcmar@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH v2 1/3] KVM: LAPIC: Make lapic timer unpinned when timer
 is injected by posted-interrupt

2019-06-06 13:31+0800, Wanpeng Li:
> From: Wanpeng Li <wanpengli@...cent.com>
> 
> Make lapic timer unpinned when timer is injected by posted-interrupt,
> the emulated timer can be offload to the housekeeping cpus.
> 
> The host admin should fine tuned, e.g. dedicated instances scenario 
> w/ nohz_full cover the pCPUs which vCPUs resident, several pCPUs 
> surplus for housekeeping, disable mwait/hlt/pause vmexits to occupy 
> the pCPUs, fortunately preemption timer is disabled after mwait is 
> exposed to guest which makes emulated timer offload can be possible. 
> 
> Cc: Paolo Bonzini <pbonzini@...hat.com>
> Cc: Radim Krčmář <rkrcmar@...hat.com>
> Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
> ---
>  arch/x86/kvm/lapic.c            | 20 ++++++++++++++++----
>  arch/x86/kvm/x86.c              |  5 +++++
>  arch/x86/kvm/x86.h              |  2 ++
>  include/linux/sched/isolation.h |  2 ++
>  kernel/sched/isolation.c        |  6 ++++++
>  5 files changed, 31 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index fcf42a3..09b7387 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -127,6 +127,12 @@ static inline u32 kvm_x2apic_id(struct kvm_lapic *apic)
>  	return apic->vcpu->vcpu_id;
>  }
>  
> +static inline bool posted_interrupt_inject_timer_enabled(struct kvm_vcpu *vcpu)
> +{
> +	return pi_inject_timer && kvm_vcpu_apicv_active(vcpu) &&
> +		kvm_mwait_in_guest(vcpu->kvm);

I'm torn about the mwait dependency.  It covers a lot of the targeted
user base, but the relation is convoluted and not fitting perfectly.

What do you think about making posted_interrupt_inject_timer_enabled()
just

	pi_inject_timer && kvm_vcpu_apicv_active(vcpu)

and disarming the vmx preemption timer when
posted_interrupt_inject_timer_enabled(), just like we do with mwait now?

Thanks.