linux-kernel - Re: [PATCH v2 09/17] KVM: x86/mmu: Use '0' as the one and only value for an invalid PAE root

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Message-ID: <63d2a610-f897-805c-23a7-488a65485f36@redhat.com>
Date:   Fri, 5 Mar 2021 18:52:44 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Ben Gardon <bgardon@...gle.com>,
        Brijesh Singh <brijesh.singh@....com>,
        Tom Lendacky <thomas.lendacky@....com>
Subject: Re: [PATCH v2 09/17] KVM: x86/mmu: Use '0' as the one and only value
 for an invalid PAE root

On 05/03/21 02:10, Sean Christopherson wrote:
> Use '0' to denote an invalid pae_root instead of '0' or INVALID_PAGE.
> Unlike root_hpa, the pae_roots hold permission bits and thus are
> guaranteed to be non-zero.  Having to deal with both values leads to
> bugs, e.g. failing to set back to INVALID_PAGE, warning on the wrong
> value, etc...

I don't dispute this is a good idea, but it deserves one or more comments.

Paolo