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? 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
