On Tue, 24 May 2016 08:53:14 +0800 Wanpeng Li <kernellwp@xxxxxxxxx> wrote: > 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. Hi, Wanpeng, thanks for the check. Please have a look on the changes to the arch/x86/kvm/x86.c on the patch, the inject_expired_hwemul_timer() is called twice there. Of course, per Paolo's review, this code path will be removed on the next submission. Thanks --jyh > > 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
