Re: [PATCH 3/4] KVM: x86: hyper-v: Track Hyper-V TSC page status

[Vitaly Kuznetsov <vkuznets@redhat.com>]

Sean Christopherson <seanjc@google.com> writes:

> On Mon, Mar 15, 2021, Vitaly Kuznetsov wrote:
>> Create an infrastructure for tracking Hyper-V TSC page status, i.e. if it
>> was updated from guest/host side or if we've failed to set it up (because
>> e.g. guest wrote some garbage to HV_X64_MSR_REFERENCE_TSC) and there's no
>> need to retry.
>> 
>> Also, in a hypothetical situation when we are in 'always catchup' mode for
>> TSC we can now avoid contending 'hv->hv_lock' on every guest enter by
>> setting the state to HV_TSC_PAGE_BROKEN after compute_tsc_page_parameters()
>> returns false.
>> 
>> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
>> ---
>> diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
>> index eefb85b86fe8..2a8d078b16cb 100644
>> --- a/arch/x86/kvm/hyperv.c
>> +++ b/arch/x86/kvm/hyperv.c
>> @@ -1087,7 +1087,7 @@ void kvm_hv_setup_tsc_page(struct kvm *kvm,
>>  	BUILD_BUG_ON(sizeof(tsc_seq) != sizeof(hv->tsc_ref.tsc_sequence));
>>  	BUILD_BUG_ON(offsetof(struct ms_hyperv_tsc_page, tsc_sequence) != 0);
>>  
>> -	if (!(hv->hv_tsc_page & HV_X64_MSR_TSC_REFERENCE_ENABLE))
>> +	if (hv->hv_tsc_page_status == HV_TSC_PAGE_BROKEN)
>>  		return;
>>  
>>  	mutex_lock(&hv->hv_lock);
>
> ...
>
>> @@ -1133,6 +1133,12 @@ void kvm_hv_setup_tsc_page(struct kvm *kvm,
>>  	hv->tsc_ref.tsc_sequence = tsc_seq;
>>  	kvm_write_guest(kvm, gfn_to_gpa(gfn),
>>  			&hv->tsc_ref, sizeof(hv->tsc_ref.tsc_sequence));
>> +
>> +	hv->hv_tsc_page_status = HV_TSC_PAGE_SET;
>> +	goto out_unlock;
>> +
>> +out_err:
>> +	hv->hv_tsc_page_status = HV_TSC_PAGE_BROKEN;
>>  out_unlock:
>>  	mutex_unlock(&hv->hv_lock);
>>  }
>> @@ -1193,8 +1199,13 @@ static int kvm_hv_set_msr_pw(struct kvm_vcpu *vcpu, u32 msr, u64 data,
>>  	}
>>  	case HV_X64_MSR_REFERENCE_TSC:
>>  		hv->hv_tsc_page = data;
>> -		if (hv->hv_tsc_page & HV_X64_MSR_TSC_REFERENCE_ENABLE)
>> +		if (hv->hv_tsc_page & HV_X64_MSR_TSC_REFERENCE_ENABLE) {
>> +			if (!host)
>> +				hv->hv_tsc_page_status = HV_TSC_PAGE_GUEST_CHANGED;
>> +			else
>> +				hv->hv_tsc_page_status = HV_TSC_PAGE_HOST_CHANGED;
>
> Writing the status without taking hv->hv_lock could cause the update to be lost,
> e.g. if a different vCPU fails kvm_hv_setup_tsc_page() at the same time, its
> write to set status to HV_TSC_PAGE_BROKEN would race with this write.
>

Oh, right you are, the lock was somewhere in my brain :-) Will do in v2.

>>  			kvm_make_request(KVM_REQ_MASTERCLOCK_UPDATE, vcpu);
>> +		}
>>  		break;
>>  	case HV_X64_MSR_CRASH_P0 ... HV_X64_MSR_CRASH_P4:
>>  		return kvm_hv_msr_set_crash_data(kvm,
>> -- 
>> 2.30.2
>> 
>

-- 
Vitaly