Hi wangpeng, On 2017/3/16 16:51, Wanpeng Li wrote: > 2017-03-16 10:08 GMT+08:00 Longpeng (Mike) <longpeng2@xxxxxxxxxx>: >> Hi, Radim, >> >> On 2017/3/16 1:32, Radim Krčmář wrote: >> >>> 2017-03-13 15:12+0800, Longpeng (Mike): >>>> Hi guys, >>>> >>>> I'm confusing about the behavior of HLT instruction in VMX guest mode. >>>> >>>> I set "hlt exiting" bit to 0 in VMCS, and the vcpu didn't vmexit when execute >>>> HLT as expected. However, I used powertop/cpupower on host to watch the pcpu's >>>> c-states, it seems that the pcpu didn't enter C1/C1E state during this period. >>>> >>>> I searched the Intel spec vol-3, and only found that guest MWAIT won't entering >>>> a low-power sleep state under certain conditions(ch 25.3), but not mentioned HLT. >>>> >>>> My questions are >>>> 1) Does executing HLT instruction in guest-mode won't enter C1/C1E state ? >>> >>> Do you get a different result when running HLT outside VMX? >>> >> >> >> Yep, I'm sure that executing HLT in host will enter C1/C1E state, but it won't >> when executing in guest. > > Execute hlt will enter C1 state, just mwait can enter C1E state. > You're right, thanks. :) So...It is really more precise to say that executing HLT in host will enter C1 state and it won't when executing in guest if "HLT exiting = 0". > Regards, > Wanpeng Li > >> >>>> 2) If it won't, then whether it would release the hardware resources shared with >>>> another hyper-thread ? >>> >> >>> No idea. Aren't hyperthreaded resources scheduled dynamically, so even >>> a nop-spinning VCPU won't hinder the other hyper-thread? >>> >> >> >> I had wrote a testcase in kvm-unit-tests, and it seems that guest-mode HLT-ed >> vcpu won't compete the hardware resources( maybe including the pipeline ) any more. >> >> My testcase is: binding vcpu1 and vcpu2 to a core's 2 hyper-threads, and >> >> (vcpu1) >> t1 = rdtsc(); >> for (int i = 0; i < 10000000; ++i) ; >> t2 = rdtsc(); >> costs = t2 - t1; >> >> (vcpu2) >> "halt" or "while (1) ;" >> >> The result is: >> ----------------------------------------------------------------------- >> (vcpu2)idle=poll (vcpu2)idle=halt >> (HLT exiting=1) >> vcpu1 costs 3800931 1900209 >> >> (HLT exiting=0) >> vcpu1 costs 3800193 1913514 >> ----------------------------------------------------------------------- >> >> I found that https://www.spinics.net/lists/kvm-commits/msg00137.html had maked >> "HLT exiting" configurable, while >> http://lkml.iu.edu/hypermail/linux/kernel/1202.0/03309.html removed it due to >> redundant with CFS hardlimit. >> >> I focus on the VM's performance. According the result, I think running HLT in >> guest-mode is better than idle=poll with HLT-exiting in *certain* scenarios. >> >>>> Any suggestion would be greatly appreciated, thanks! >>> >>> Mostly just more questions, sorry ... >>> >>> I'd look at temperature sensors while halting inside guests on all cores >>> to see if they really enter a power saving mode -- I expect a noticeable >>> difference from idle=poll. :) >>> >>> >> >> >> -- >> Regards, >> Longpeng(Mike) >> > > . > -- Regards, Longpeng(Mike)
