Re: [PATCH v4 4/7] KVM: mmu: Add slots_arch_lock for memslot arch fields

[Sean Christopherson <seanjc@google.com>]

On Tue, May 11, 2021, Ben Gardon wrote:
> Add a new lock to protect the arch-specific fields of memslots if they
> need to be modified in a kvm->srcu read critical section. A future
> commit will use this lock to lazily allocate memslot rmaps for x86.
> 
> Signed-off-by: Ben Gardon <bgardon@google.com>
> ---
>  include/linux/kvm_host.h |  9 +++++++++
>  virt/kvm/kvm_main.c      | 31 ++++++++++++++++++++++++++-----
>  2 files changed, 35 insertions(+), 5 deletions(-)
> 
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 8895b95b6a22..2d5e797fbb08 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -472,6 +472,15 @@ struct kvm {
>  #endif /* KVM_HAVE_MMU_RWLOCK */
>  
>  	struct mutex slots_lock;
> +
> +	/*
> +	 * Protects the arch-specific fields of struct kvm_memory_slots in
> +	 * use by the VM. To be used under the slots_lock (above) or in a
> +	 * kvm->srcu read cirtical section where acquiring the slots_lock
                          ^^^^^^^^
			  critical

> +	 * would lead to deadlock with the synchronize_srcu in
> +	 * install_new_memslots.
> +	 */
> +	struct mutex slots_arch_lock;
>  	struct mm_struct *mm; /* userspace tied to this vm */
>  	struct kvm_memslots __rcu *memslots[KVM_ADDRESS_SPACE_NUM];
>  	struct kvm_vcpu *vcpus[KVM_MAX_VCPUS];