linux-kernel - Re: [PATCH v2 06/18] KVM: x86/mmu: do not consult levels when freeing roots

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Date:   Wed, 23 Feb 2022 16:59:26 +0200
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     seanjc@...gle.com
Subject: Re: [PATCH v2 06/18] KVM: x86/mmu: do not consult levels when
 freeing roots

On Thu, 2022-02-17 at 16:03 -0500, 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
> 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 may hold PAE roots, as root.hpa isn't backed by a shadow when
> it points at PAE roots.  Using this method results in simpler code, and
> is one less obstacle in moving all calls to __kvm_mmu_new_pgd() after the
> MMU has been initialized.
> 
> 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 a478667d7561..e1578f71feae 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -3240,12 +3240,15 @@ 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);
> +
> +	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))
> @@ -3263,8 +3266,7 @@ 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) {

Makes sense, although that will collide hard with that RFC of backing all shadow pages
with kvm mmu pages.

Reviewed-by: Maxim Levitsky <mlevitsk@...hat.com>

Best regards,
	Maxim Levitsky