linux-kernel - Re: [PATCH] KVM: VMX: switch to hrtimer for TSC deadline timer when L2 guest is running

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Date:	Wed, 6 Jul 2016 16:01:46 +0800
From:	Haozhong Zhang <haozhong.zhang@...el.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	Wanpeng Li <kernellwp@...il.com>, kvm <kvm@...r.kernel.org>,
	Radim Krcmar <rkrcmar@...hat.com>,
	Yunhong Jiang <yunhong.jiang@...el.com>,
	Wanpeng Li <wanpeng.li@...mail.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Jan Kiszka <jan.kiszka@...mens.com>
Subject: Re: [PATCH] KVM: VMX: switch to hrtimer for TSC deadline timer when
 L2 guest is running

On 07/06/16 09:46, Paolo Bonzini wrote:
> 
> 
> On 06/07/2016 08:05, Haozhong Zhang wrote:
> >> > 
> >> > Nested preemption timer is emulated by hrtimer, so it doesn't
> >> > influence vmcs02, why this is needed?
> > Nested (L2) preemption timer is not affected for sure, and this patch
> > is to fix another problem caused by using L1 preemption timer for L1
> > TSC deadline timer. When we use L1 VMX preemption timer for L1 TSC
> > deadline timer, we intend to use the pin-based exec config and the VMX
> > preemption timer value in vmcs01. However, when L2 guest is running,
> > vmcs02 is loaded as the current VMCS so that a different VMX
> > preemption timer config is used (i.e. VMX preemption timer is disabled
> > in prepare_vmcs02()). If we still use preemption timer for L1 TSC
> > deadline timer at this moment, then L1 TSC deadline timer will not be
> > able to be triggered when L2 guest is running.
> 
> The right fix then is to set the pin-based controls in prepare_vmcs02,
> based on vcpu->arch.hv_deadline_tsc.

Then KVM needs to distinguish L1 and L2 VMEXITs for preemption timer
(or does KVM already have this?). The current implementation which
uses a separate hrtimer for L2 preemption timer can easily distinguish
them.

Haozhong