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

[Paolo Bonzini]

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);
> 		}
> 	}