Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Subject: Re: [PATCH] KVM: SVM: Make sure GHCB is mapped before updating
To:     Tom Lendacky <thomas.lendacky@amd.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>
 <YG4fAeaTy0HdHCsT@google.com> <b1a6dddd-9485-c6f6-8af9-79f6e3505206@amd.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <2b282772-afaa-2fd4-0794-4449eda6fd02@redhat.com>
Date:   Thu, 8 Apr 2021 18:14:27 +0200
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101
 Thunderbird/78.7.0
MIME-Version: 1.0
In-Reply-To: <b1a6dddd-9485-c6f6-8af9-79f6e3505206@amd.com>
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Precedence: bulk

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

Paolo