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Subject: Re: [PATCH] KVM: SVM: Make sure GHCB is mapped before updating
To:     Paolo Bonzini <pbonzini@redhat.com>,
        Sean Christopherson <seanjc@google.com>
Cc:     kvm@vger.kernel.org, linux-kernel@vger.kernel.org, x86@kernel.org,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Borislav Petkov <bp@alien8.de>, Ingo Molnar <mingo@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Brijesh Singh <brijesh.singh@amd.com>
References: <03b349cb19b360d4c2bbeebdd171f99298082d28.1617820214.git.thomas.lendacky@amd.com>
 <YG4RSl88TSPccRfj@google.com> <d46ee7c3-6c8c-1f06-605c-c4f2d1888ba4@amd.com>
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 <2b282772-afaa-2fd4-0794-4449eda6fd02@redhat.com>
From:   Tom Lendacky <thomas.lendacky@amd.com>
Message-ID: <8654d5eb-e6d0-af94-db46-7a4fce8caa0d@amd.com>
Date:   Thu, 8 Apr 2021 11:18:43 -0500
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On 4/8/21 11:14 AM, Paolo Bonzini wrote:
> On 08/04/21 18:04, Tom Lendacky wrote:
>>>>> +       if (!err || !sev_es_guest(vcpu->kvm) ||
>>>>> !WARN_ON_ONCE(svm->ghcb))
>>>> This should be WARN_ON_ONCE(!svm->ghcb), otherwise you'll get the right
>>>> result, but get a stack trace immediately.
>>> Doh, yep.
>> Actually, because of the "or's", this needs to be:
>>
>> if (!err || !sev_es_guest(vcpu->kvm) || (sev_es_guest(vcpu->kvm) &&
>> WARN_ON_ONCE(!svm->ghcb)))
> 
> No, || cuts the right-hand side if the left-hand side is true.  So:
> 
> - if err == 0, the rest is not evaluated
> 
> - if !sev_es_guest(vcpu->kvm), WARN_ON_ONCE(!svm->ghcb) is not evaluated

That's what I was doing in my head, but I guess I need more coffee... :)

Thanks,
Tom

> 
> Paolo
>