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

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Date:   Thu, 10 May 2018 21:40:17 +0200
From:   Radim Krčmář <rkrcmar@...hat.com>
To:     Vitaly Kuznetsov <vkuznets@...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

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?