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: <4EF1B8A4.2050008@redhat.com>
Date:	Wed, 21 Dec 2011 12:44:52 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Jan Kiszka <jan.kiszka@...mens.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 12/21/2011 12:41 PM, Jan Kiszka wrote:
> 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.
>

timer_mode_mask can just be slaved to the bit (as received by
KVM_SET_CPUID); that's exactly kvm_update_cpuid()'s role.

-- 
error compiling committee.c: too many arguments to function

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