Re: [PATCH v2 35/43] KVM: SVM: Signal AVIC doorbell iff vCPU is in guest mode

Paolo Bonzini <[email protected]> Wed, 27 Oct 2021 15:36:18 +0000

Newsgroups	org.kernel.vger.kvm-ppc,dev.linux.lists.kvmarm,org.infradead.lists.kvm-riscv,org.infradead.lists.linux-arm-kernel,org.infradead.lists.linux-riscv,org.kernel.vger.kvm,org.kernel.vger.linux-kernel,org.kernel.vger.linux-mips
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On 27/10/21 17:06, Sean Christopherson wrote:
>> Does this still need to check the "running" flag?  That should be a strict
>> superset of vcpu->mode = IN_GUEST_MODE.
>
> No.  Signalling the doorbell when "running" is set but the vCPU is not in the
> guest is just an expensive nop.  So even if KVM were to rework its handling of
> "running" to set the flag immediately before VMRUN and clear it immediately after,
> keying off IN_GUEST_MODE and not "running" would not be wrong, just sub-optimal.
> 
> I doubt KVM will ever make the "running" flag super precise, because keeping the
> flag set when the vCPU is loaded avoids VM-Exits on other vCPUs due to undelivered
> IPIs.

Right, so should we drop the "if (running)" check in this patch, at the 
same time as it's adding the IN_GUEST_MODE check?

Paolo