linux-kernel - Re: [PATCH 3/5] KVM: x86: hyperv: simplistic HVCALL_FLUSH_VIRTUAL_ADDRESS

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Date:   Wed, 04 Apr 2018 11:27:22 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Radim Krčmář <rkrcmar@...hat.com>
Cc:     kvm@...r.kernel.org, x86@...nel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Roman Kagan <rkagan@...tuozzo.com>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "Michael Kelley \(EOSG\)" <Michael.H.Kelley@...rosoft.com>,
        Mohammed Gamal <mmorsy@...hat.com>,
        Cathy Avery <cavery@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/5] KVM: x86: hyperv: simplistic HVCALL_FLUSH_VIRTUAL_ADDRESS_{LIST,SPACE} implementation

Radim Krčmář <rkrcmar@...hat.com> writes:

> 2018-04-02 18:10+0200, Vitaly Kuznetsov:
>> Implement HvFlushVirtualAddress{List,Space} hypercalls in a simplistic way:
>> do full TLB flush with KVM_REQ_TLB_FLUSH and rely on kvm_vcpu_kick()
>> kicking only vCPUs which are currently IN_GUEST_MODE.
>> 
>> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
>> ---
>>  arch/x86/kvm/hyperv.c | 54 ++++++++++++++++++++++++++++++++++++++++++++-------
>>  arch/x86/kvm/trace.h  | 24 +++++++++++++++++++++++
>>  2 files changed, 71 insertions(+), 7 deletions(-)
>> 
>> diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
>> index 3cb3bb68db7e..aa866994366d 100644
>> --- a/arch/x86/kvm/hyperv.c
>> +++ b/arch/x86/kvm/hyperv.c
>> @@ -1242,6 +1242,49 @@ int kvm_hv_get_msr_common(struct kvm_vcpu *vcpu, u32 msr, u64 *pdata)
>>  		return kvm_hv_get_msr(vcpu, msr, pdata);
>>  }
>>  
>> +static u64 kvm_hv_flush_tlb(struct kvm_vcpu *current_vcpu, u64 ingpa,
>> +			    u16 rep_cnt)
>> +{
>> +	struct kvm *kvm = current_vcpu->kvm;
>> +	struct hv_tlb_flush flush;
>> +	struct kvm_vcpu *vcpu;
>> +	int i;
>> +
>> +	if (unlikely(kvm_read_guest(kvm, ingpa, &flush, sizeof(flush))))
>> +		return HV_STATUS_INVALID_HYPERCALL_INPUT;
>> +
>> +	trace_kvm_hv_flush_tlb(flush.processor_mask, flush.address_space,
>> +			       flush.flags);
>> +
>> +	kvm_for_each_vcpu(i, vcpu, kvm) {
>> +		struct kvm_vcpu_hv *hv = &vcpu->arch.hyperv;
>> +
>> +		if (!(flush.flags & HV_FLUSH_ALL_PROCESSORS) &&
>> +		    !(flush.processor_mask & BIT_ULL(hv->vp_index)))
>> +			continue;
>> +
>> +		/*
>> +		 * vcpu->arch.cr3 may not be up-to-date for running vCPUs so we
>> +		 * can't analyze it here, flush TLB regardless of the specified
>> +		 * address space.
>> +		 */
>> +		kvm_make_request(KVM_REQ_TLB_FLUSH, vcpu);
>> +
>> +		/*
>> +		 * It is very unlikely but possible that we're doing an extra
>> +		 * kick here (e.g. if the vCPU has just entered the guest and
>> +		 * has its TLB flushed).
>> +		 */
>> +		if (vcpu != current_vcpu)
>> +			kvm_vcpu_kick(vcpu);
>
> The spec says that
>
>  "This call guarantees that by the time control returns back to the
>   caller, the observable effects of all flushes on the specified virtual
>   processors have occurred."
>
> Other KVM code doesn't assume that kvm_vcpu_kick() and a delay provides
> that guarantee;  kvm_make_all_cpus_request waits for the target CPU to
> exit before saying that TLB has been flushed.
>
> I am leaning towards the safer variant here as well.  (Anyway, it's a
> good time to figure out if we really need it.)

Ha, it depends on how we define "observable effects" :-)

I think kvm_vcpu_kick() is enough as the corresponding vCPU can't
actually observe old mapping after being kicked (even if we didn't flush
yet we're not running). Or do you see any possible problem with such
definition?


>
>> +	}
>> +
>> +	/* We always do full TLB flush, set rep_done = rep_cnt. */
>> +	return (u64)HV_STATUS_SUCCESS |
>> +		((u64)rep_cnt << HV_HYPERCALL_REP_START_OFFSET) |
>
> Why at bits 48-59?  I don't see this field in the spec.
>

True, it is only for 'input'. Will drop.


>> +		((u64)rep_cnt << HV_HYPERCALL_REP_COMP_OFFSET);
>> +}
>> +
>>  bool kvm_hv_hypercall_enabled(struct kvm *kvm)
>>  {
>>  	return READ_ONCE(kvm->arch.hyperv.hv_hypercall) & HV_X64_MSR_HYPERCALL_ENABLE;
>> @@ -1345,12 +1388,6 @@ int kvm_hv_hypercall(struct kvm_vcpu *vcpu)
>>  
>>  	trace_kvm_hv_hypercall(code, fast, rep_cnt, rep_idx, ingpa, outgpa);
>>  
>> -	/* Hypercall continuation is not supported yet */
>> -	if (rep_cnt || rep_idx) {
>> -		ret = HV_STATUS_INVALID_HYPERCALL_CODE;
>
> Hm, we should have returned HV_STATUS_INVALID_HYPERCALL_INPUT in any
> case.  I think it would be good to still fail in case of non-rep
> hypercalls,

Sure. I skimmed through the spec and didn't find any direct reference
that specifying 'rep' bit for non-rep hypercalls is forbidden but this
is definitely a guest bug if it does that. 

Thanks for the review!

-- 
  Vitaly