linux-kernel - Re: [PATCH 5/6] KVM: x86/mmu: Protect kvm->memslots with a mutex

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Message-ID: <997f9fe3-847b-8216-c629-1ad5fdd2ffae@redhat.com>
Date:   Wed, 28 Apr 2021 08:25:20 +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>,
        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 5/6] KVM: x86/mmu: Protect kvm->memslots with a mutex

On 28/04/21 00:36, Ben Gardon wrote:
> +void kvm_arch_assign_memslots(struct kvm *kvm, int as_id,
> +			     struct kvm_memslots *slots)
> +{
> +	mutex_lock(&kvm->arch.memslot_assignment_lock);
> +	rcu_assign_pointer(kvm->memslots[as_id], slots);
> +	mutex_unlock(&kvm->arch.memslot_assignment_lock);
> +}

Does the assignment also needs the lock, or only the rmap allocation?  I 
would prefer the hook to be something like kvm_arch_setup_new_memslots.

(Also it is useful to have a comment somewhere explaining why the 
slots_lock does not work.  IIUC there would be a deadlock because you'd 
be taking the slots_lock inside an SRCU critical region, while usually 
the slots_lock critical section is the one that includes a 
synchronize_srcu; I should dig that up and document that ordering in 
Documentation/virt/kvm too).

Paolo