linux-kernel - Re: [PATCH 08/13] KVM: x86/mmu: Protect the tdp_mmu

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Date:   Thu, 1 Apr 2021 11:37:12 +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 08/13] KVM: x86/mmu: Protect the tdp_mmu_roots list with
 RCU

On 31/03/21 23:08, Ben Gardon wrote:
> Protect the contents of the TDP MMU roots list with RCU in preparation
> for a future patch which will allow the iterator macro to be used under
> the MMU lock in read mode.
> 
> Signed-off-by: Ben Gardon<bgardon@...gle.com>
> ---
>   arch/x86/kvm/mmu/tdp_mmu.c | 64 +++++++++++++++++++++-----------------
>   1 file changed, 36 insertions(+), 28 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> +	spin_lock(&kvm->arch.tdp_mmu_pages_lock);
> +	list_del_rcu(&root->link);
> +	spin_unlock(&kvm->arch.tdp_mmu_pages_lock);


Please update the comment above tdp_mmu_pages_lock in 
arch/x86/include/asm/kvm_host.h as well.

>  /* Only safe under the MMU lock in write mode, without yielding. */
>  #define for_each_tdp_mmu_root(_kvm, _root)				\
> -	list_for_each_entry(_root, &_kvm->arch.tdp_mmu_roots, link)
> +	list_for_each_entry_rcu(_root, &_kvm->arch.tdp_mmu_roots, link,	\
> +				lockdep_is_held_write(&kvm->mmu_lock))

This should also add "... || 
lockdep_is_help(&kvm->arch.tdp_mmu_pages_lock)", if only for 
documentation purposes.

Paolo