linux-kernel - Re: [PATCH v2] KVM: x86: Prevent exposing TSC deadline timer feature in the absence of in-kernel APIC

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Message-ID: <4EF1B7C0.3080200@siemens.com>
Date:	Wed, 21 Dec 2011 11:41:04 +0100
From:	Jan Kiszka <jan.kiszka@...mens.com>
To:	Avi Kivity <avi@...hat.com>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Alexey Zaytsev <alexey.zaytsev@...il.com>,
	"Liu, Jinsong" <jinsong.liu@...el.com>,
	Kernel development list <linux-kernel@...r.kernel.org>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	"Garrett D'Amore" <garrett@...enta.com>, kvm <kvm@...r.kernel.org>
Subject: Re: [PATCH v2] KVM: x86: Prevent exposing TSC deadline timer feature
 in the absence of in-kernel APIC

On 2011-12-21 11:35, Avi Kivity wrote:
> On 12/21/2011 12:25 PM, Jan Kiszka wrote:
>> We must not report the TSC deadline timer feature on our own when user
>> space provides the APIC as we have no clue about its features.
> 
> We must not report the TSC deadline timer feature on our own, period. 
> We should just update the timer mode mask there.  Don't know how this
> slipped through review.
>
> I think your original idea was correct.  Add a new KVM_CAP for the tsc
> deadline timer.  Userspace can add the bit to cpuid if either it
> implements the feature in a userspace apic, or if it finds the new
> capability and uses the kernel apic.

Right, we do need some control for user space to keep the feature
disabled when migrating from an older host.

However, there is also the timer_mode_mask which requires tuning in
addition to the cpuid flag.

Jan

-- 
Siemens AG, Corporate Technology, CT T DE IT 1
Corporate Competence Center Embedded Linux
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