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

[Vitaly Kuznetsov <vkuznets@redhat.com>]

Vitaly Kuznetsov <vkuznets@redhat.com> writes:

> 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.

Actually no, kvm_hv_set_msr_pw() is only called from
kvm_hv_set_msr_common() with hv->hv_lock held so we're already
synchronized.

... and of course I figured that our by putting another
mutex_lock()/mutex_unlock() here and then wondering why everything hangs
:-)

>
>>>  			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