linux-kernel - Re: [PATCH 19/19] Revert "KVM: SVM: Do not throw warning when calling avic_vcpu

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Date:   Wed, 31 Aug 2022 09:07:11 +0300
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Suravee Suthikulpanit <suravee.suthikulpanit@....com>,
        Li RongQing <lirongqing@...du.com>
Subject: Re: [PATCH 19/19] Revert "KVM: SVM: Do not throw warning when
 calling avic_vcpu_load on a running vcpu"

On Wed, 2022-08-31 at 00:35 +0000, Sean Christopherson wrote:
> Turns out that some warnings exist for good reasons.  Restore the warning
> in avic_vcpu_load() that guards against calling avic_vcpu_load() on a
> running vCPU now that KVM avoids doing so when switching between x2APIC
> and xAPIC.  The entire point of the WARN is to highlight that KVM should
> not be reloading an AVIC.
> 
> Opportunistically convert the WARN_ON() to WARN_ON_ONCE() to avoid
> spamming the kernel if it does fire.
> 
> This reverts commit c0caeee65af3944b7b8abbf566e7cc1fae15c775.
> 
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  arch/x86/kvm/svm/avic.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
> index b2033a56010c..3c300113d40b 100644
> --- a/arch/x86/kvm/svm/avic.c
> +++ b/arch/x86/kvm/svm/avic.c
> @@ -1080,6 +1080,7 @@ void avic_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
>  		return;
>  
>  	entry = READ_ONCE(*(svm->avic_physical_id_cache));
> +	WARN_ON_ONCE(entry & AVIC_PHYSICAL_ID_ENTRY_IS_RUNNING_MASK);
>  
>  	entry &= ~AVIC_PHYSICAL_ID_ENTRY_HOST_PHYSICAL_ID_MASK;
>  	entry |= (h_physical_id & AVIC_PHYSICAL_ID_ENTRY_HOST_PHYSICAL_ID_MASK);

Note that this warning was removed because it would trigger wheh x2avic code would switch
between xapic and x2apic.

I do agree 100% that this warning is useful.

Best regards,
	Maxim Levitsky