Re: [PATCH v6 4/7] KVM: x86: hyperv: keep track of mismatched VP indexes

[Paolo Bonzini]

On 27/09/2018 11:17, Vitaly Kuznetsov wrote:
> Roman Kagan <rkagan@xxxxxxxxxxxxx> writes:
> 
>> On Wed, Sep 26, 2018 at 07:02:56PM +0200, Vitaly Kuznetsov wrote:
>>> In most common cases VP index of a vcpu matches its vcpu index. Userspace
>>> is, however, free to set any mapping it wishes and we need to account for
>>> that when we need to find a vCPU with a particular VP index. To keep search
>>> algorithms optimal in both cases introduce 'num_mismatched_vp_indexes'
>>> counter showing how many vCPUs with mismatching VP index we have. In case
>>> the counter is zero we can assume vp_index == vcpu_idx.
>>>
>>> Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
>>> ---
>>>  arch/x86/include/asm/kvm_host.h |  3 +++
>>>  arch/x86/kvm/hyperv.c           | 26 +++++++++++++++++++++++---
>>>  2 files changed, 26 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
>>> index 09b2e3e2cf1b..711f79f1b5e6 100644
>>> --- a/arch/x86/include/asm/kvm_host.h
>>> +++ b/arch/x86/include/asm/kvm_host.h
>>> @@ -781,6 +781,9 @@ struct kvm_hv {
>>>  	u64 hv_reenlightenment_control;
>>>  	u64 hv_tsc_emulation_control;
>>>  	u64 hv_tsc_emulation_status;
>>> +
>>> +	/* How many vCPUs have VP index != vCPU index */
>>> +	atomic_t num_mismatched_vp_indexes;
>>>  };
>>>  
>>>  enum kvm_irqchip_mode {
>>> diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
>>> index c8764faf783b..6a19c8e3c432 100644
>>> --- a/arch/x86/kvm/hyperv.c
>>> +++ b/arch/x86/kvm/hyperv.c
>>> @@ -1045,11 +1045,31 @@ static int kvm_hv_set_msr(struct kvm_vcpu *vcpu, u32 msr, u64 data, bool host)
>>>  	struct kvm_vcpu_hv *hv_vcpu = &vcpu->arch.hyperv;
>>>  
>>>  	switch (msr) {
>>> -	case HV_X64_MSR_VP_INDEX:
>>> -		if (!host || (u32)data >= KVM_MAX_VCPUS)
>>> +	case HV_X64_MSR_VP_INDEX: {
>>> +		struct kvm_hv *hv = &vcpu->kvm->arch.hyperv;
>>> +		int vcpu_idx = kvm_vcpu_get_idx(vcpu);
>>> +		u32 new_vp_index = (u32)data;
>>> +
>>> +		if (!host || new_vp_index >= KVM_MAX_VCPUS)
>>>  			return 1;
>>> -		hv_vcpu->vp_index = (u32)data;
>>> +
>>> +		if (new_vp_index == hv_vcpu->vp_index)
>>> +			return 0;
>>> +
>>> +		/*
>>> +		 * VP index is changing, increment num_mismatched_vp_indexes in
>>> +		 * case it was equal to vcpu_idx before; on the other hand, if
>>> +		 * the new VP index matches vcpu_idx num_mismatched_vp_indexes
>>> +		 * needs to be decremented.
>>
>> It may be worth mentioning that the initial balance is provided by
>> kvm_hv_vcpu_postcreate setting vp_index = vcpu_idx.
>>
> 
> Of course, yes, will update the comment in case I'll be re-submitting.

	/*
	 * VP index is initialized to hv_vcpu->vp_index by
	 * kvm_hv_vcpu_postcreate so they initially match.  Now the
	 * VP index is changing, adjust num_mismatched_vp_indexes if
	 * it now matches or no longer matches vcpu_idx.
	 */

?

Paolo