linux-kernel - Re: [PATCH RFC V6 1/5] kvm hypervisor : Add a hypercall to KVM hypervisor to support pv-ticketlocks

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Date:	Thu, 28 Jun 2012 23:47:27 +0530
From:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
To:	Gleb Natapov <gleb@...hat.com>, Avi Kivity <avi@...hat.com>
CC:	Jeremy Fitzhardinge <jeremy@...p.org>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	Alexander Graf <agraf@...e.de>,
	Randy Dunlap <rdunlap@...otime.net>, linux-doc@...r.kernel.org,
	"H. Peter Anvin" <hpa@...or.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	KVM <kvm@...r.kernel.org>,
	Stefano Stabellini <stefano.stabellini@...citrix.com>,
	Virtualization <virtualization@...ts.linux-foundation.org>,
	X86 <x86@...nel.org>, Ingo Molnar <mingo@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Xen <xen-devel@...ts.xensource.com>,
	Sasha Levin <levinsasha928@...il.com>,
	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>
Subject: Re: [PATCH RFC V6 1/5] kvm hypervisor : Add a hypercall to KVM hypervisor
 to support pv-ticketlocks

On 04/27/2012 09:23 PM, Gleb Natapov wrote:
> On Fri, Apr 27, 2012 at 04:15:35PM +0530, Raghavendra K T wrote:
>> On 04/24/2012 03:29 PM, Gleb Natapov wrote:
>>> On Mon, Apr 23, 2012 at 03:29:47PM +0530, Raghavendra K T wrote:
>>>> From: Srivatsa Vaddagiri<vatsa@...ux.vnet.ibm.com>
>>>>
>>>> KVM_HC_KICK_CPU allows the calling vcpu to kick another vcpu out of halt state.
>>>>
>>>> The presence of these hypercalls is indicated to guest via
>>>> KVM_FEATURE_PV_UNHALT/KVM_CAP_PV_UNHALT.
>>>>
>>>> Signed-off-by: Srivatsa Vaddagiri<vatsa@...ux.vnet.ibm.com>
>>>> Signed-off-by: Suzuki Poulose<suzuki@...ibm.com>
>>>> Signed-off-by: Raghavendra K T<raghavendra.kt@...ux.vnet.ibm.com>
>>>> ---
>> [...]
>>>> +/*
>>>> + * kvm_pv_kick_cpu_op:  Kick a vcpu.
>>>> + *
>>>> + * @apicid - apicid of vcpu to be kicked.
>>>> + */
>>>> +static void kvm_pv_kick_cpu_op(struct kvm *kvm, int apicid)
>>>> +{
>>>> +	struct kvm_vcpu *vcpu = NULL;
>>>> +	int i;
>>>> +
>>>> +	kvm_for_each_vcpu(i, vcpu, kvm) {
>>>> +		if (!kvm_apic_present(vcpu))
>>>> +			continue;
>>>> +
>>>> +		if (kvm_apic_match_dest(vcpu, 0, 0, apicid, 0))
>>>> +			break;
>>>> +	}
>>>> +	if (vcpu) {
>>>> +		/*
>>>> +		 * Setting unhalt flag here can result in spurious runnable
>>>> +		 * state when unhalt reset does not happen in vcpu_block.
>>>> +		 * But that is harmless since that should soon result in halt.
>>>> +		 */
>>>> +		vcpu->arch.pv.pv_unhalted = 1;
>>>> +		/* We need everybody see unhalt before vcpu unblocks */
>>>> +		smp_mb();
>>>> +		kvm_vcpu_kick(vcpu);
>>>> +	}
>>>> +}
>>> This is too similar to kvm_irq_delivery_to_apic(). Why not reuse it. We
>>> can use one of reserved delivery modes as PV delivery mode. We will
>>> disallow guest to trigger it through apic interface, so this will not be
>>> part of ABI and can be changed at will.
[...]
>> kvm/x86.c
>> =========
>> kvm_pv_kick_cpu_op()
>> {
>>
>>   struct kvm_lapic_irq lapic_irq;
>>
>>   lapic_irq.shorthand = 0;
>>   lapic_irq.dest_mode = 0;
>>   lapic_irq.dest_id = apicid;
>>
>>   lapic_irq.delivery_mode = PV_DELIVERY_MODE;
>>   kvm_irq_delivery_to_apic(kvm, 0,&lapic_irq );
>>
>> }
>>
>> kvm/lapic.c
>> ==========
>> _apic_accept_irq()
>> {
>> ...
>> case APIC_DM_REMRD:
>>                  result = 1;
>>                  vcpu->pv_unhalted = 1
>>                  smp_mb();
>>                  kvm_make_request(KVM_REQ_EVENT, vcpu);
>>                  kvm_vcpu_kick(vcpu);
>>                  break;
>>
>> ...
>> }
>>
>> here using PV_DELIVERY_MODE = APIC_DM_REMRD, which was unused.
>>
> Yes, this is what I mean except that PV_DELIVERY_MODE should be
> number defined as reserved by Intel spec.
>

Hi Gleb, Avi,

This had been TODO in my V8 patches.
I 'll fold this into V9 (while rebasing to
3.5-rc).
Please let me know if it is OK.

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