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From: "Radim Krčmář" <rkrcmar@redhat.com>
To: "Longpeng (Mike)" <longpeng2@huawei.com>
Cc: kvm <kvm@vger.kernel.org>
Subject: Re: [Question]  About the behavior of HLT in VMX guest mode
Date: Wed, 15 Mar 2017 18:32:54 +0100	[thread overview]
Message-ID: <20170315173254.GF14081@potion> (raw)
In-Reply-To: <58C64672.1070706@huawei.com>

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?

> 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?

> 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. :)

  reply	other threads:[~2017-03-15 17:43 UTC|newest]

Thread overview: 20+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-03-13  7:12 [Question] About the behavior of HLT in VMX guest mode Longpeng (Mike)
2017-03-15 17:32 ` Radim Krčmář [this message]
2017-03-16  2:08   ` Longpeng (Mike)
2017-03-16  2:48     ` Longpeng (Mike)
2017-03-16  8:51     ` Wanpeng Li
2017-03-16  9:19       ` Longpeng (Mike)
2017-03-16 14:23     ` Radim Krčmář
2017-03-17  5:22       ` Longpeng (Mike)
2017-03-20 15:18         ` Radim Krčmář
2017-03-21  1:47           ` Longpeng (Mike)
2017-03-21  6:21             ` Wanpeng Li
2017-03-21 16:45               ` Radim Krčmář
2017-07-04  4:24                 ` Wanpeng Li
2017-07-10 17:08                   ` Radim Krčmář
2017-07-11 10:41                     ` Wanpeng Li
2017-07-11 15:04                       ` Radim Krčmář
  -- strict thread matches above, loose matches on Subject: below --
2017-03-13  6:12 Longpeng (Mike)
2017-03-13  5:12 Longpeng (Mike)
2017-03-13 11:38 ` Jan Beulich
2017-03-14  1:11   ` Longpeng (Mike)

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