linux-kernel - Re: [RFC PATCH 1/9] kvm_main.c: move slot check in kvm_set_memory

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Date:   Wed, 28 Sep 2022 18:41:00 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Emanuele Giuseppe Esposito <eesposit@...hat.com>,
        kvm@...r.kernel.org
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        David Hildenbrand <david@...hat.com>,
        Maxim Levitsky <mlevitsk@...hat.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 1/9] kvm_main.c: move slot check in
 kvm_set_memory_region

On 9/9/22 12:44, Emanuele Giuseppe Esposito wrote:
> And make kvm_set_memory_region static, since it is not used outside
> kvm_main.c
> 
> Signed-off-by: Emanuele Giuseppe Esposito <eesposit@...hat.com>
> ---
>   include/linux/kvm_host.h |  2 --
>   virt/kvm/kvm_main.c      | 11 +++++------
>   2 files changed, 5 insertions(+), 8 deletions(-)
> 
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 3b40f8d68fbb..1c5b7b2e35dd 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -1108,8 +1108,6 @@ enum kvm_mr_change {
>   	KVM_MR_FLAGS_ONLY,
>   };
>   
> -int kvm_set_memory_region(struct kvm *kvm,
> -			  const struct kvm_userspace_memory_region *mem);
>   int __kvm_set_memory_region(struct kvm *kvm,
>   			    const struct kvm_userspace_memory_region *mem);
>   void kvm_arch_free_memslot(struct kvm *kvm, struct kvm_memory_slot *slot);
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index da263c370d00..339de0ed4557 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -2007,24 +2007,23 @@ int __kvm_set_memory_region(struct kvm *kvm,
>   }
>   EXPORT_SYMBOL_GPL(__kvm_set_memory_region);
>   
> -int kvm_set_memory_region(struct kvm *kvm,
> -			  const struct kvm_userspace_memory_region *mem)
> +static int kvm_set_memory_region(struct kvm *kvm,
> +				 const struct kvm_userspace_memory_region *mem)
>   {
>   	int r;
>   
> +	if ((u16)mem->slot >= KVM_USER_MEM_SLOTS)
> +		return -EINVAL;
> +
>   	mutex_lock(&kvm->slots_lock);
>   	r = __kvm_set_memory_region(kvm, mem);
>   	mutex_unlock(&kvm->slots_lock);
>   	return r;
>   }
> -EXPORT_SYMBOL_GPL(kvm_set_memory_region);
>   
>   static int kvm_vm_ioctl_set_memory_region(struct kvm *kvm,
>   					  struct kvm_userspace_memory_region *mem)
>   {
> -	if ((u16)mem->slot >= KVM_USER_MEM_SLOTS)
> -		return -EINVAL;
> -
>   	return kvm_set_memory_region(kvm, mem);
>   }
>   

The idea here was that kvm_set_memory_region could be used to set 
private memory slots while not taking kvm->slots_lock.

So, I would instead:

1) rename __kvm_set_memory_region to kvm_set_memory_region;

2) inline the old kvm_set_memory_region into kvm_vm_ioctl_set_memory_region.

3) replace the comment "Must be called holding kvm->slots_lock for 
write." with a proper lockdep_assert_held() now that the function 
doesn't have the __ warning sign in front of it.

Paolo