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: <YEJ21vvQfBXnvlP8@google.com>
Date:   Fri, 5 Mar 2021 10:22:14 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.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 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))


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);
		}
	}