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

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Message-ID: <YgWwrG+EQgTwyt8v@google.com>
Date:   Fri, 11 Feb 2022 00:41:16 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        vkuznets@...hat.com, mlevitsk@...hat.com, dmatlack@...gle.com
Subject: Re: [PATCH 08/12] KVM: MMU: do not consult levels when freeing roots

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@...hat.com>
> ---
>  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) {
>  			for (i = 0; i < 4; ++i) {
> -- 
> 2.31.1
> 
>