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:	Wed, 21 Dec 2011 12:28:29 +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 11:44, Avi Kivity wrote:
> 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.

Like

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 230f713..89b02bf 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -27,7 +27,6 @@ void kvm_update_cpuid(struct kvm_vcpu *vcpu)
 {
 	struct kvm_cpuid_entry2 *best;
 	struct kvm_lapic *apic = vcpu->arch.apic;
-	u32 timer_mode_mask;

 	best = kvm_find_cpuid_entry(vcpu, 1, 0);
 	if (!best)
@@ -40,15 +39,12 @@ void kvm_update_cpuid(struct kvm_vcpu *vcpu)
 			best->ecx |= bit(X86_FEATURE_OSXSAVE);
 	}

-	if (boot_cpu_data.x86_vendor == X86_VENDOR_INTEL &&
-		best->function == 0x1) {
-		best->ecx |= bit(X86_FEATURE_TSC_DEADLINE_TIMER);
-		timer_mode_mask = 3 << 17;
-	} else
-		timer_mode_mask = 1 << 17;
-
-	if (apic)
-		apic->lapic_timer.timer_mode_mask = timer_mode_mask;
+	if (apic) {
+		if (best->ecx & bit(X86_FEATURE_TSC_DEADLINE_TIMER))
+			apic->lapic_timer.timer_mode_mask = 3 << 17;
+		else
+			apic->lapic_timer.timer_mode_mask = 1 << 17;
+	}

 	kvm_pmu_cpuid_update(vcpu);
 }


+ the KVM_CAP thing?

Jan

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