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

From: Paolo Bonzini <pbonzini@redhat.com>
To: Sean Christopherson <seanjc@google.com>
Cc: Vitaly Kuznetsov <vkuznets@redhat.com>,
	Wanpeng Li <wanpengli@tencent.com>,
	Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>,
	kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
	Ben Gardon <bgardon@google.com>,
	Brijesh Singh <brijesh.singh@amd.com>,
	Tom Lendacky <thomas.lendacky@amd.com>
Subject: Re: [PATCH v2 09/17] KVM: x86/mmu: Use '0' as the one and only value for an invalid PAE root
Date: Fri, 5 Mar 2021 19:23:18 +0100	[thread overview]
Message-ID: <c4e386f6-35f5-fdd7-10c9-c690615f1a47@redhat.com> (raw)
In-Reply-To: <YEJ21vvQfBXnvlP8@google.com>

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);
> 		}
> 	}
> 