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

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Date:	Thu, 22 Dec 2011 15:41:52 +0000
From:	"Liu, Jinsong" <jinsong.liu@...el.com>
To:	Avi Kivity <avi@...hat.com>, Jan Kiszka <jan.kiszka@...mens.com>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Alexey Zaytsev <alexey.zaytsev@...il.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

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.

Is it necessary to use KVM_CAP? If I didn't misunderstand, the KVM_CAP sulotion would be:
1. qemu get kvm tsc deadline timer capability by KVM_CAP_...;
2. qemu add cpuid bit
        if ((guest use qemu apic && qemu emualte tsc deadline timer) || 
           (guest use kvm apic && kvm emulate tsc deadline timer (KVM_CAP)))
3. qemu ioctl KVM_SET_CPUID2
4. kvm expose the feature to guest by saving it at vcpu->arch.cpuid_entries, 
seems it's logically redundant.

Jan's patch v2 is a straight forward and simple fix. in the patch
    if (apic) { ... }
means apic (and then its sub-logic tsc deadline timer) emulated by kvm, that's enough:
if quest use kvm apic, it's OK to add cpuid bit and expose to guest;
if guest don't use kvm apic, it will not touch cpuid bit;

Thanks,
Jinsong--
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