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SubjectRe: [PATCH] KVM: Avoid atomic operations when kicking the running vCPU
From
On 20/10/21 21:34, Sean Christopherson wrote:
>>
>> + /*
>> + * The only state change done outside the vcpu mutex is IN_GUEST_MODE
>> + * to EXITING_GUEST_MODE. Therefore the moderately expensive "should
>> + * kick" check does not need atomic operations if kvm_vcpu_kick is used
>> + * within the vCPU thread itself.
>> + */
>> + if (vcpu == __this_cpu_read(kvm_running_vcpu)) {
>> + if (vcpu->mode == IN_GUEST_MODE)
>> + WRITE_ONCE(vcpu->mode, EXITING_GUEST_MODE);
>
> Fun. I had a whole thing typed out about this being unsafe because it implicitly
> relies on a pending request and that there's a kvm_vcpu_exit_request() check _after_
> this kick. Then I saw your other patches, and then I realized we already have this
> bug in the kvm_arch_vcpu_should_kick() below.

Yeah, the three patches are independent but part of the same rabbit hole.

> Anyways, I also think we should add do:
>
> if (vcpu == __this_cpu_read(kvm_running_vcpu)) {
> if (vcpu->mode == IN_GUEST_MODE &&
> !WARN_ON_ONCE(!kvm_request_pending(vcpu)))
> WRITE_ONCE(vcpu->mode, EXITING_GUEST_MODE);
> goto out;
> }
>
> The idea being that delaying or even missing an event in case of a KVM bug is
> preferable to letting the vCPU state become invalid due to running in the guest
> with EXITING_GUEST_MODE.

On one hand I like the idea of having a safety net; for example a test
similar to this one would have triggered for the naked
kvm_vcpu_exiting_guest_mode(vcpu) call in vmx_sync_pir_to_irr.

On the other hand, "request-less VCPU kicks", as
Documentation/virt/kvm/vcpu-requests.rst calls them, are a thing; PPC
book3s_hv does not use vcpu->requests at all. For an artificial but more
relatable example, the ON bit takes the role of vcpu->requests when
processing PIR. Hence the code below would be suboptimal but still correct:

for (;;) {
exit_fastpath = static_call(kvm_x86_run)(vcpu);
if (likely(exit_fastpath !=
EXIT_FASTPATH_REENTER_GUEST))
break;

if (vcpu->arch.apicv_active && pi_test_on(vcpu))
kvm_vcpu_kick(vcpu);

if (unlikely(kvm_vcpu_exit_request(vcpu))) {
exit_fastpath = EXIT_FASTPATH_EXIT_HANDLED;
break;
}
}

All that really matters is that every call to kvm_x86_run is guarded by
kvm_vcpu_exit_request(vcpu), and indeed that's what is restored by "KVM:
x86: check for interrupts before deciding whether to exit the fast
path". The other architectures also have similar checks, though again
it's a bit hard to find it for book3s_hv (due to not using
vcpu->requests) and MIPS (which only uses KVM_REQ_TLB_FLUSH).

Paolo

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