Re: [PATCH 08/12] KVM: MMU: do not consult levels when freeing roots

[Sean Christopherson]

On Fri, Feb 11, 2022, Sean Christopherson wrote:
> On Wed, Feb 09, 2022, Paolo Bonzini wrote:
> > Right now, PGD caching requires a complicated dance of first computing
> > the MMU role and passing it to __kvm_mmu_new_pgd, and then separately calling
> 
> Nit, adding () after function names helps readers easily recognize when you're
> taking about a specific function, e.g. as opposed to a concept or whatever.
> 
> > kvm_init_mmu.
> > 
> > Part of this is due to kvm_mmu_free_roots using mmu->root_level and
> > mmu->shadow_root_level to distinguish whether the page table uses a single
> > root or 4 PAE roots.  Because kvm_init_mmu can overwrite mmu->root_level,
> > kvm_mmu_free_roots must be called before kvm_init_mmu.
> > 
> > However, even after kvm_init_mmu there is a way to detect whether the page table
> > has a single root or four, because the pae_root does not have an associated
> > struct kvm_mmu_page.
> 
> Suggest a reword on the final paragraph, because there's a discrepancy with the
> code (which handles 0, 1, or 4 "roots", versus just "single or four").
> 
>   However, even after kvm_init_mmu() there is a way to detect whether the
>   page table may hold PAE roots, as root.hpa isn't backed by a shadow when
>   it points at PAE roots.
> 
> > Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> > ---
> >  arch/x86/kvm/mmu/mmu.c | 10 ++++++----
> >  1 file changed, 6 insertions(+), 4 deletions(-)
> > 
> > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > index 3c3f597ea00d..95d0fa0bb876 100644
> > --- a/arch/x86/kvm/mmu/mmu.c
> > +++ b/arch/x86/kvm/mmu/mmu.c
> > @@ -3219,12 +3219,15 @@ void kvm_mmu_free_roots(struct kvm_vcpu *vcpu, struct kvm_mmu *mmu,
> >  	struct kvm *kvm = vcpu->kvm;
> >  	int i;
> >  	LIST_HEAD(invalid_list);
> > -	bool free_active_root = roots_to_free & KVM_MMU_ROOT_CURRENT;
> > +	bool free_active_root;
> >  
> >  	BUILD_BUG_ON(KVM_MMU_NUM_PREV_ROOTS >= BITS_PER_LONG);
> >  
> >  	/* Before acquiring the MMU lock, see if we need to do any real work. */
> > -	if (!(free_active_root && VALID_PAGE(mmu->root.hpa))) {
> > +	free_active_root = (roots_to_free & KVM_MMU_ROOT_CURRENT)
> > +		&& VALID_PAGE(mmu->root.hpa);
> 
> 	free_active_root = (roots_to_free & KVM_MMU_ROOT_CURRENT) &&
> 			   VALID_PAGE(mmu->root.hpa);
> 
> Isn't this a separate bug fix?  E.g. call kvm_mmu_unload() without a valid current
> root, but with valid previous roots?  In which case we'd try to free garbage, no?
> 			   
> > +
> > +	if (!free_active_root) {
> >  		for (i = 0; i < KVM_MMU_NUM_PREV_ROOTS; i++)
> >  			if ((roots_to_free & KVM_MMU_ROOT_PREVIOUS(i)) &&
> >  			    VALID_PAGE(mmu->prev_roots[i].hpa))
> > @@ -3242,8 +3245,7 @@ void kvm_mmu_free_roots(struct kvm_vcpu *vcpu, struct kvm_mmu *mmu,
> >  					   &invalid_list);
> >  
> >  	if (free_active_root) {
> > -		if (mmu->shadow_root_level >= PT64_ROOT_4LEVEL &&
> > -		    (mmu->root_level >= PT64_ROOT_4LEVEL || mmu->direct_map)) {
> > +		if (to_shadow_page(mmu->root.hpa)) {
> >  			mmu_free_root_page(kvm, &mmu->root.hpa, &invalid_list);
> >  		} else if (mmu->pae_root) {

Gah, this is technically wrong.  It shouldn't truly matter, but it's wrong.  root.hpa
will not be backed by shadow page if the root is pml4_root or pml5_root, in which
case freeing the PAE root is wrong.  They should obviously be invalid already, but
it's a little confusing because KVM wanders down a path that may not be relevant
to the current mode.

For clarity, I think it's worth doing:

		} else if (mmu->root.hpa == __pa(mmu->pae_root)) {


> >  			for (i = 0; i < 4; ++i) {
> > -- 
> > 2.31.1
> > 
> >