linux-kernel - Re: [PATCH v2 09/13] KVM: x86/mmu: Allow zap gfn range to operate under the mmu read lock

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Message-ID: <4fc5960f-0b64-1cf5-d2c1-080d82d226a0@redhat.com>
Date:   Fri, 2 Apr 2021 09:53:08 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Ben Gardon <bgardon@...gle.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     Peter Xu <peterx@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Peter Shier <pshier@...gle.com>,
        Peter Feiner <pfeiner@...gle.com>,
        Junaid Shahid <junaids@...gle.com>,
        Jim Mattson <jmattson@...gle.com>,
        Yulei Zhang <yulei.kernel@...il.com>,
        Wanpeng Li <kernellwp@...il.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Xiao Guangrong <xiaoguangrong.eric@...il.com>
Subject: Re: [PATCH v2 09/13] KVM: x86/mmu: Allow zap gfn range to operate
 under the mmu read lock

On 02/04/21 01:37, Ben Gardon wrote:
> +void kvm_tdp_mmu_put_root(struct kvm *kvm, struct kvm_mmu_page *root,
> +			  bool shared)
>   {
>   	gfn_t max_gfn = 1ULL << (shadow_phys_bits - PAGE_SHIFT);
>   
> -	lockdep_assert_held_write(&kvm->mmu_lock);
> +	kvm_lockdep_assert_mmu_lock_held(kvm, shared);
>   
>   	if (!refcount_dec_and_test(&root->tdp_mmu_root_count))
>   		return;
> @@ -81,7 +92,7 @@ void kvm_tdp_mmu_put_root(struct kvm *kvm, struct kvm_mmu_page *root)
>   	list_del_rcu(&root->link);
>   	spin_unlock(&kvm->arch.tdp_mmu_pages_lock);
>   
> -	zap_gfn_range(kvm, root, 0, max_gfn, false, false);
> +	zap_gfn_range(kvm, root, 0, max_gfn, false, false, shared);
>   
>   	call_rcu(&root->rcu_head, tdp_mmu_free_sp_rcu_callback);

Instead of patch 13, would it make sense to delay the zap_gfn_range and 
call_rcu to a work item (either unconditionally, or only if 
shared==false)?  Then the zap_gfn_range would be able to yield and take 
the mmu_lock for read, similar to kvm_tdp_mmu_zap_invalidated_roots.

If done unconditionally, this would also allow removing the "shared" 
argument to kvm_tdp_mmu_put_root, tdp_mmu_next_root and 
for_each_tdp_mmu_root_yield_safe, so I would place that change before 
this patch.

Paolo