linux-kernel - Re: [PATCH v3 4/6] KVM: x86: hyperv: simplistic HVCALL_FLUSH_VIRTUAL_ADDRESS

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Date:   Fri, 11 May 2018 14:27:10 +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 v3 4/6] KVM: x86: hyperv: simplistic HVCALL_FLUSH_VIRTUAL_ADDRESS_{LIST,SPACE} implementation

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

> 2018-04-16 13:08+0200, Vitaly Kuznetsov:
>> Implement HvFlushVirtualAddress{List,Space} hypercalls in a simplistic way:
>> do full TLB flush with KVM_REQ_TLB_FLUSH and kick vCPUs which are currently
>> IN_GUEST_MODE.
>> 
>> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
>> ---
>> diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
>> @@ -1242,6 +1242,65 @@ int kvm_hv_get_msr_common(struct kvm_vcpu *vcpu, u32 msr, u64 *pdata)
>>  		return kvm_hv_get_msr(vcpu, msr, pdata);
>>  }
>>  
>> +static void ack_flush(void *_completed)
>> +{
>> +}
>> +
>> +static u64 kvm_hv_flush_tlb(struct kvm_vcpu *current_vcpu, u64 ingpa,
>> +			    u16 rep_cnt)
>> +{
>> +	struct kvm *kvm = current_vcpu->kvm;
>> +	struct kvm_vcpu_hv *hv_current = &current_vcpu->arch.hyperv;
>> +	struct hv_tlb_flush flush;
>> +	struct kvm_vcpu *vcpu;
>> +	int i, cpu, me;
>> +
>> +	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);
>> +
>> +	cpumask_clear(&hv_current->tlb_lush);
>> +
>> +	me = get_cpu();
>> +
>> +	kvm_for_each_vcpu(i, vcpu, kvm) {
>> +		struct kvm_vcpu_hv *hv = &vcpu->arch.hyperv;
>> +
>> +		if (!(flush.flags & HV_FLUSH_ALL_PROCESSORS) &&
>
> Please add a check to prevent undefined behavior in C:
>
>                     (hv->vp_index >= 64 ||
>
>> +		    !(flush.processor_mask & BIT_ULL(hv->vp_index)))
>> +			continue;
>
> It would also fail in the wild as shl only considers the bottom 5 bits.
>
>> +		/*
>> +		 * 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 possible that vCPU will migrate and we will kick wrong
>> +		 * CPU but vCPU's TLB will anyway be flushed upon migration as
>> +		 * we already made KVM_REQ_TLB_FLUSH request.
>> +		 */
>> +		cpu = vcpu->cpu;
>> +		if (cpu != -1 && cpu != me && cpu_online(cpu) &&
>> +		    kvm_arch_vcpu_should_kick(vcpu))
>> +			cpumask_set_cpu(cpu, &hv_current->tlb_lush);
>> +	}
>> +
>> +	if (!cpumask_empty(&hv_current->tlb_lush))
>> +		smp_call_function_many(&hv_current->tlb_lush, ack_flush,
>> +				       NULL, true);
>
> Hm, quite a lot of code duplication with EX hypercall and also
> kvm_make_all_cpus_request ... I'm thinking about making something like
>
>   kvm_make_some_cpus_request(struct kvm *kvm, unsigned int req,
>                              bool (*predicate)(struct kvm_vcpu *vcpu))
>
> or to implement a vp_index -> vcpu mapping and using
>
>   kvm_vcpu_request_mask(struct kvm *kvm, unsigned int req, long *vcpu_bitmap)
>
> The latter would probably simplify logic of the EX hypercall.
>
> What do you think?

Makes sense, I'll take a look. Thanks!

-- 
  Vitaly