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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Date:   Fri, 5 Mar 2021 19:23:18 +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 19:22, Sean Christopherson wrote:
> On Fri, Mar 05, 2021, Paolo Bonzini wrote:
>> 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.
> 
> Agreed.   What about adding macros?
> 
> /* Comment goes here. */
> #define INVALID_PAE_ROOT	0
> #define IS_VALID_PAE_ROOT(x)	(!!(x))

Yep, that's a nice solution.

Paolo

> 
> 
> Also, I missed this pattern in mmu_audit.c's mmu_spte_walk():
> 
> 	for (i = 0; i < 4; ++i) {
> 		hpa_t root = vcpu->arch.mmu->pae_root[i];
> 
> 		if (root && VALID_PAGE(root)) {
> 			root &= PT64_BASE_ADDR_MASK;
> 			sp = to_shadow_page(root);
> 			__mmu_spte_walk(vcpu, sp, fn, 2);
> 		}
> 	}
>