Re: [PATCH v3 4/6] KVM: x86: hyperv: simplistic HVCALL_FLUSH_VIRTUAL_ADDRESS_{LIST,SPACE} implementation

Vitaly Kuznetsov
2018-05-13T08:47:12Z

Radim Krčmář <rkrcmar@redhat.com> writes:

> 2018-04-16 13:08+0200, Vitaly Kuznetsov:
...
>
>> + /*
>> + * 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.

We really want to avoid memory allocation for cpumask on this path and
that's what kvm_make_all_cpus_request() currently does (when
CPUMASK_OFFSTACK). vcpu bitmap is probably OK as KVM_MAX_VCPUS is much
lower.

Making cpumask allocation avoidable leads us to the following API:

bool kvm_make_vcpus_request_mask(struct kvm *kvm, unsigned int req,
long *vcpu_bitmap, cpumask_var_t tmp);

or, if we want to prettify this a little bit, we may end up with the
following pair:

bool kvm_make_vcpus_request_mask(struct kvm *kvm, unsigned int req,
long *vcpu_bitmap);

bool __kvm_make_vcpus_request_mask(struct kvm *kvm, unsigned int req,
long *vcpu_bitmap, cpumask_var_t tmp);

and from hyperv code we'll use the later. With this, no code duplication
is required.

Does this look acceptable?

--
Vitaly