> -----Original Message----- > From: kvm-owner@xxxxxxxxxxxxxxx [mailto:kvm-owner@xxxxxxxxxxxxxxx] On > Behalf Of Paolo Bonzini > Sent: Friday, May 20, 2016 1:49 PM > To: Marcelo Tosatti <mtosatti@xxxxxxxxxx> > Cc: Yunhong Jiang <yunhong.jiang@xxxxxxxxxxxxxxx>; kvm@xxxxxxxxxxxxxxx; > rkrcmar@xxxxxxxxxx > Subject: Re: [RFC PATCH 0/5] Utilizing VMX preemption for timer > virtualization > > > On Thu, May 19, 2016 at 06:44:58PM -0700, Yunhong Jiang wrote: > > > The VMX-preemption timer is a feature on VMX, it counts down, from > the > > > value loaded by VM entry, in VMX nonroot operation. When the timer > > > counts down to zero, it stops counting down and a VM exit occurs. > > > > > > The VMX preemption timer for tsc deadline timer virtualization. The > > > VMX preemption timer is armed when the vCPU is running, and a VMExit > > > will happen if the virtual TSC deadline timer expires. > > > > > > When the vCPU thread is scheduled out, the tsc deadline timer > > > virtualization will be switched to use the current solution, i.e. use > > > the timer for it. It's switched back to VMX preemption timer when the > > > vCPU thread is scheduled int. > > > > > > This solution replace the complex OS's hrtimer system, and also the > > > host timer interrupt handling cost, with a preemption_timer VMexit. It > > > fits well for some NFV usage scenario, when the vCPU is bound to a > > > pCPU and the pCPU is isolated, or some similar scenarioes. > > > > > > However, it possibly has impact if the vCPU thread is scheduled in/out > > > very frequently, because it switches from/to the hrtimer emulation a > > > lot. A module parameter is provided to turn it on or off. > > > > > > Signed-off-by: Yunhong Jiang <yunhong.jiang@xxxxxxxxx> > > > > Hi Yunhong Jiang, > > > > This adds cost to the VM-exit and VM-entry paths (additional > > instructions and i-cache pressure). Also it adds cost to > > kvm_sched_out. > > For now my review limited itself to making the code nicer without > touching the overall design too much. > > I'm confident that we can reduce it to a dozen instructions on vmentry If we set the preemption timer on the first vm-entry, and then there is a vm-exit because of another reason, on the second vm-entry, do we need update the VMX_PREEMPTION_TIMER_VALUE value? If no, then we don't need anything special on the vmentry. We simply set the VMCS in sched_in or when guest program the timer. Thanks --jyh > and only pay the cost of hrtimer_start on failed HLT polls (enabling > the hrtimer only before going to sleep). Assuming that device > interrupts are delivered while the guest is running or during a > successful HLT polls, that would be very rare. > > > What is the benefit the switch from external interrupt to VMX preemption > > timer brings? > > hrtimer_start/hrtimer_cancel do show up in profiles for dynticks guests. > Since they touch a red-black tree, it shouldn't be hard to outperform them. > > Paolo > -- > To unsubscribe from this list: send the line "unsubscribe kvm" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html ��.n��������+%������w��{.n�����o�^n�r������&��z�ޗ�zf���h���~����������_��+v���)ߣ�
