2016-05-24 6:58 GMT+08:00 yunhong jiang <yunhong.jiang@xxxxxxxxxxxxxxx>: > On Sun, 22 May 2016 08:21:50 +0800 > Wanpeng Li <kernellwp@xxxxxxxxx> wrote: > >> 2016-05-21 6:06 GMT+08:00 Jiang, Yunhong <yunhong.jiang@xxxxxxxxx>: >> > >> > >> >> -----Original Message----- >> >> From: kvm-owner@xxxxxxxxxxxxxxx [mailto:kvm-owner@xxxxxxxxxxxxxxx] >> >> On Behalf Of Paolo Bonzini >> >> Sent: Friday, May 20, 2016 3:34 AM >> >> To: Yunhong Jiang <yunhong.jiang@xxxxxxxxxxxxxxx>; >> >> kvm@xxxxxxxxxxxxxxx Cc: rkrcmar@xxxxxxxxxx >> >> Subject: Re: [RFC PATCH 4/5] Utilize the vmx preemption timer for >> >> tsc deadline timer >> >> >> >> >> >> >> >> On 20/05/2016 03:45, Yunhong Jiang wrote: >> >> > From: Yunhong Jiang <yunhong.jiang@xxxxxxxxx> >> >> > >> >> > Utilizing 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 avoids 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 >> >> > scenario. >> >> > >> >> > 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. >> >> > >> >> > Signed-off-by: Yunhong Jiang <yunhong.jiang@xxxxxxxxx> >> >> > --- >> >> > arch/x86/kvm/lapic.c | 108 >> >> +++++++++++++++++++++++++++++++++++++++++++++++++-- >> >> > arch/x86/kvm/lapic.h | 10 +++++ >> >> > arch/x86/kvm/vmx.c | 26 +++++++++++++ >> >> > arch/x86/kvm/x86.c | 6 +++ >> >> > 4 files changed, 147 insertions(+), 3 deletions(-) >> >> > >> >> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c >> >> > index 5776473be362..a613bcfda59a 100644 >> >> > --- a/arch/x86/kvm/x86.c >> >> > +++ b/arch/x86/kvm/x86.c >> >> > @@ -6608,6 +6608,8 @@ static int vcpu_enter_guest(struct kvm_vcpu >> >> *vcpu) >> >> > >> >> > local_irq_disable(); >> >> > >> >> > + inject_expired_hwemul_timer(vcpu); >> >> >> >> Is this really fast enough (and does it trigger often enough) that >> >> it is worth slowing down all vmenters? >> >> >> >> I'd rather call inject_expired_hwemul_timer from the preemption >> >> timer vmexit handler instead. inject_pending_hwemul_timer will >> >> set the preemption timer countdown to zero if the deadline of the >> >> guest LAPIC timer has passed already. This should be relatively >> >> rare. >> > >> > Sure and will take this way on the new patch set. I'd give some >> > reson why it's this way now. Originally this patch was for >> > cyclictest on guest with latency less than 15us for 24 hours. So, >> > if the timer expires already before VM entry, we try to inject it >> > immediately, instead of waiting for an extra VMExit, which may be >> > 4~5 us. >> >> inject_expired_hwemul_timer() just set the pending bit, and still need >> a vmexit to final exit to vcpu_run() which is the only place to check >> pending and inject APIC_LVTT, so why add inject_expired_hwemul_timer() >> in vcpu_enter_guest() can avoid an extra vmexit? > > The inject_expired_hwemul_timer() will invoke the kvm_make_request(), which > will cause the kvm try to inject the timer interrupt directly. Please notice > the vcpu_enter_guest() will recheck the requests late. Actually I did't find another place inject pending timer irqs except in vcpu_run, though kvm_set_pending_timer mention that there is implicitly checked in vcpu_enter_guest. Regards, Wanpeng Li -- 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
