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Message-ID: <ecb9a22e-fd6e-67f0-d916-ad16033fc13c@csail.mit.edu>
Date: Fri, 20 Jan 2023 05:55:11 -0800
From: "Srivatsa S. Bhat" <srivatsa@...il.mit.edu>
To: Thomas Gleixner <tglx@...utronix.de>,
Igor Mammedov <imammedo@...hat.com>
Cc: linux-kernel@...r.kernel.org, amakhalov@...are.com,
ganb@...are.com, ankitja@...are.com, bordoloih@...are.com,
keerthanak@...are.com, blamoreaux@...are.com, namit@...are.com,
Peter Zijlstra <peterz@...radead.org>,
Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
Dave Hansen <dave.hansen@...ux.intel.com>,
"H. Peter Anvin" <hpa@...or.com>,
"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
"Paul E. McKenney" <paulmck@...nel.org>,
Wyes Karny <wyes.karny@....com>,
Lewis Caroll <lewis.carroll@....com>,
Tom Lendacky <thomas.lendacky@....com>,
Juergen Gross <jgross@...e.com>, x86@...nel.org,
VMware PV-Drivers Reviewers <pv-drivers@...are.com>,
virtualization@...ts.linux-foundation.org, kvm@...r.kernel.org,
xen-devel@...ts.xenproject.org
Subject: Re: [PATCH v2] x86/hotplug: Do not put offline vCPUs in mwait idle
state
Hi Igor and Thomas,
Thank you for your review!
On 1/19/23 1:12 PM, Thomas Gleixner wrote:
> On Mon, Jan 16 2023 at 15:55, Igor Mammedov wrote:
>> "Srivatsa S. Bhat" <srivatsa@...il.mit.edu> wrote:
>>> Fix this by preventing the use of mwait idle state in the vCPU offline
>>> play_dead() path for any hypervisor, even if mwait support is
>>> available.
>>
>> if mwait is enabled, it's very likely guest to have cpuidle
>> enabled and using the same mwait as well. So exiting early from
>> mwait_play_dead(), might just punt workflow down:
>> native_play_dead()
>> ...
>> mwait_play_dead();
>> if (cpuidle_play_dead()) <- possible mwait here
>> hlt_play_dead();
>>
>> and it will end up in mwait again and only if that fails
>> it will go HLT route and maybe transition to VMM.
>
> Good point.
>
>> Instead of workaround on guest side,
>> shouldn't hypervisor force VMEXIT on being uplugged vCPU when it's
>> actually hot-unplugging vCPU? (ex: QEMU kicks vCPU out from guest
>> context when it is removing vCPU, among other things)
>
> For a pure guest side CPU unplug operation:
>
> guest$ echo 0 >/sys/devices/system/cpu/cpu$N/online
>
> the hypervisor is not involved at all. The vCPU is not removed in that
> case.
>
Agreed, and this is indeed the scenario I was targeting with this patch,
as opposed to vCPU removal from the host side. I'll add this clarification
to the commit message.
> So to ensure that this ends up in HLT something like the below is
> required.
>
> Note, the removal of the comment after mwait_play_dead() is intentional
> because the comment is completely bogus. Not having MWAIT is not a
> failure. But that wants to be a seperate patch.
>
Sounds good, will do and post a new version.
Thank you!
Regards,
Srivatsa
VMware Photon OS
> Thanks,
>
> tglx
> ---
> diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
> index 55cad72715d9..3f1f20f71ec5 100644
> --- a/arch/x86/kernel/smpboot.c
> +++ b/arch/x86/kernel/smpboot.c
> @@ -1833,7 +1833,10 @@ void native_play_dead(void)
> play_dead_common();
> tboot_shutdown(TB_SHUTDOWN_WFS);
>
> - mwait_play_dead(); /* Only returns on failure */
> + if (this_cpu_has(X86_FEATURE_HYPERVISOR))
> + hlt_play_dead();
> +
> + mwait_play_dead();
> if (cpuidle_play_dead())
> hlt_play_dead();
> }
>
>
>
>
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