linux-kernel - Re: [PATCH 14/14] KVM: x86: Invoke kvm_vcpu_block() directly for non-HALTED wait states

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Message-ID: <YVOT3gj1ulBTNSw3@google.com>
Date:   Tue, 28 Sep 2021 22:14:54 +0000
From:   David Matlack <dmatlack@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Marc Zyngier <maz@...nel.org>, Huacai Chen <chenhuacai@...nel.org>,
        Aleksandar Markovic <aleksandar.qemu.devel@...il.com>,
        Paul Mackerras <paulus@...abs.org>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        James Morse <james.morse@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        David Hildenbrand <david@...hat.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        linux-mips@...r.kernel.org, kvm@...r.kernel.org,
        kvm-ppc@...r.kernel.org, linux-kernel@...r.kernel.org,
        Jing Zhang <jingzhangos@...gle.com>
Subject: Re: [PATCH 14/14] KVM: x86: Invoke kvm_vcpu_block() directly for
 non-HALTED wait states

On Fri, Sep 24, 2021 at 05:55:28PM -0700, Sean Christopherson wrote:
> Call kvm_vcpu_block() directly for all wait states except HALTED so that
> kvm_vcpu_halt() is no longer a misnomer on x86.
> 
> Functionally, this means KVM will never attempt halt-polling or adjust
> vcpu->halt_poll_ns for INIT_RECEIVED (a.k.a. Wait-For-SIPI (WFS)) or
> AP_RESET_HOLD; UNINITIALIZED is handled in kvm_arch_vcpu_ioctl_run(),
> and x86 doesn't use any other "wait" states.
> 
> As mentioned above, the motivation of this is purely so that "halt" isn't
> overloaded on x86, e.g. in KVM's stats.  Skipping halt-polling for WFS
> (and RESET_HOLD) has no meaningful effect on guest performance as there
> are typically single-digit numbers of INIT-SIPI sequences per AP vCPU,
> per boot, versus thousands of HLTs just to boot to console.
> 
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>

Reviewed-by: David Matlack <dmatlack@...gle.com>

> ---
>  arch/x86/kvm/x86.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index b444f9315766..a0f313c4bc49 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -9893,7 +9893,10 @@ static inline int vcpu_block(struct kvm *kvm, struct kvm_vcpu *vcpu)
>  	if (!kvm_arch_vcpu_runnable(vcpu) &&
>  	    (!kvm_x86_ops.pre_block || static_call(kvm_x86_pre_block)(vcpu) == 0)) {
>  		srcu_read_unlock(&kvm->srcu, vcpu->srcu_idx);
> -		kvm_vcpu_halt(vcpu);
> +		if (vcpu->arch.mp_state == KVM_MP_STATE_HALTED)
> +			kvm_vcpu_halt(vcpu);
> +		else
> +			kvm_vcpu_block(vcpu);
>  		vcpu->srcu_idx = srcu_read_lock(&kvm->srcu);
>  
>  		if (kvm_x86_ops.post_block)
> -- 
> 2.33.0.685.g46640cef36-goog
>