linux-kernel - Re: [PATCH] KVM: SVM: Assume a 64-bit hypercall for guests with protected state

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Date:   Mon, 24 May 2021 19:06:44 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Tom Lendacky <thomas.lendacky@....com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Wanpeng Li <wanpengli@...cent.com>,
        Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Brijesh Singh <brijesh.singh@....com>,
        Ashish Kalra <Ashish.Kalra@....com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCH] KVM: SVM: Assume a 64-bit hypercall for guests with
 protected state

On 24/05/21 19:03, Sean Christopherson wrote:
>> Let's introduce a new wrapper is_64_bit_hypercall, and add a
>> WARN_ON_ONCE(vcpu->arch.guest_state_protected) to is_64_bit_mode.
>
> Can we introduce the WARN(s) in a separate patch, and deploy them much more
> widely than just is_64_bit_mode()?  I would like to have them lying in wait at
> every path that should be unreachable, e.g. get/set segments, get_cpl(), etc...

Each WARN that is added must be audited separately, so this one I'd like 
to have now; it is pretty much the motivation for introducing a new 
function, as the other caller of is_64_bit_mode, kvm_get_linear_rip() is 
already "handling" SEV-ES by always returning 0.

But yes adding more WARNs can only be good.

Paolo

> Side topic, kvm_get_cs_db_l_bits() should be moved to svm.c.  Functionally, it's
> fine to have it as a vendor-agnostic helper, but practically speaking it should
> never be called directly.
>