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SubjectRe: [PATCH 3/5] KVM: x86: hyperv: simplistic HVCALL_FLUSH_VIRTUAL_ADDRESS_{LIST,SPACE} implementation
Vitaly Kuznetsov <> writes:

> Radim Krčmář <> writes:
>> 2018-04-02 18:10+0200, Vitaly Kuznetsov:
>>> + 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?

Oh, now I see it myself -- native_smp_send_reschedule() only does
apic->send_IPI() so this is indeed unsafe. We need something like
kvm_make_all_cpus_request() with a mask (and, to make it fast, we'll
probably have to pre-allocate these).

Will do in v2, thanks!


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