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

Thanks Avi, for the review.

On 04/29/2012 06:55 PM, Avi Kivity wrote:
> On 04/23/2012 12:59 PM, Raghavendra K T wrote:
>> From: Srivatsa Vaddagiri<vatsa@linux.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.
>>
>> #endif
>> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
>> index e216ba0..dad475b 100644
>> --- a/arch/x86/include/asm/kvm_host.h
>> +++ b/arch/x86/include/asm/kvm_host.h
>> @@ -481,6 +481,10 @@ struct kvm_vcpu_arch {
>> u64 length;
>> u64 status;
>> } osvw;
>> + /* pv related host specific info */
>> + struct {
>> + int pv_unhalted;
>> + } pv;
>> };
>
> 'bool'. Or maybe push into vcpu->requests.

Ok. I think you meant
+ struct {
+ bool pv_unhalted;
+ } pv;

and as discussed in old series (V4), cleaner implementation having
vcpu request, would still need a flag to prevent vcpu hang, so back to
having one flag.

>
>>
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index 4044ce0..7fc9be6 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -2147,6 +2147,7 @@ int kvm_dev_ioctl_check_extension(long ext)
>> case KVM_CAP_ASYNC_PF:
>> case KVM_CAP_GET_TSC_KHZ:
>> case KVM_CAP_PCI_2_3:
>> + case KVM_CAP_PV_UNHALT:
>> r = 1;
>> break;
>> case KVM_CAP_COALESCED_MMIO:
>
> Redundant, since we can infer this from KVM_GET_SUPPORTED_CPUID. But
> please indicate this in the documentation.
>

Ok. will mention that in documentation added for KVM_CAP_PV_UNHALT.

>>
>> +/*
>> + * 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();
>
> smp_wmb().
>

Done.

>> + kvm_vcpu_kick(vcpu);
>> + }
>> +}
>> +
>>
>> /*
>> * hypercalls use architecture specific
>> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
>> index 42b7393..edf56d4 100644
>> --- a/virt/kvm/kvm_main.c
>> +++ b/virt/kvm/kvm_main.c
>> @@ -1500,6 +1500,14 @@ void kvm_vcpu_block(struct kvm_vcpu *vcpu)
>> prepare_to_wait(&vcpu->wq,&wait, TASK_INTERRUPTIBLE);
>>
>> if (kvm_arch_vcpu_runnable(vcpu)) {
>> + /*
>> + * This is the only safe place to reset unhalt flag.
>> + * otherwise it results in loosing the notification
>> + * which eventually can result in vcpu hangs.
>> + */
>> + kvm_arch_vcpu_reset_pv_unhalted(vcpu);
>> + /* preventing reordering should be enough here */
>> + barrier();
>> kvm_make_request(KVM_REQ_UNHALT, vcpu);
>> break;
>> }
>>
>
> Hm, what about reusing KVM_REQ_UNHALT?
>

Yes, I had experimented this for some time without success.
For e.g. having
make_request(KVM_REQ_UNHALT, vcpu) directly from kick hypercall.

It would still need a flag. (did not get any alternative so far except
the workaround posted in V4) :(



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