linux-kernel - Re: [PATCH v2 2/2] kvm/vfio: avoid bouncing the mutex when adding and deleting groups

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Date:   Fri, 14 Jul 2023 13:31:56 -0600
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Dmitry Torokhov <dmitry.torokhov@...il.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Sean Christopherson <seanjc@...gle.com>,
        Roxana Bradescu <roxabee@...gle.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/2] kvm/vfio: avoid bouncing the mutex when adding
 and deleting groups

On Fri, 14 Jul 2023 11:37:57 -0700
Dmitry Torokhov <dmitry.torokhov@...il.com> wrote:

> Stop taking kv->lock mutex in kvm_vfio_update_coherency() and instead
> call it with this mutex held: the callers of the function usually
> already have it taken (and released) before calling
> kvm_vfio_update_coherency(). This avoid bouncing the lock up and down.
> 
> The exception is kvm_vfio_release() where we do not take the lock, but
> it is being executed when the very last reference to kvm_device is being
> dropped, so there are no concerns about concurrency.
> 
> Suggested-by: Alex Williamson <alex.williamson@...hat.com>
> Signed-off-by: Dmitry Torokhov <dmitry.torokhov@...il.com>
> ---
> 
> v2: new patch.
> 
>  virt/kvm/vfio.c | 25 +++++++++----------------
>  1 file changed, 9 insertions(+), 16 deletions(-)
> 
> diff --git a/virt/kvm/vfio.c b/virt/kvm/vfio.c
> index cd46d7ef98d6..9868e7ccb5fb 100644
> --- a/virt/kvm/vfio.c
> +++ b/virt/kvm/vfio.c
> @@ -122,8 +122,6 @@ static void kvm_vfio_update_coherency(struct kvm_device *dev)
>  	bool noncoherent = false;
>  	struct kvm_vfio_group *kvg;
>  
> -	mutex_lock(&kv->lock);
> -
>  	list_for_each_entry(kvg, &kv->group_list, node) {
>  		if (!kvm_vfio_file_enforced_coherent(kvg->file)) {
>  			noncoherent = true;
> @@ -139,8 +137,6 @@ static void kvm_vfio_update_coherency(struct kvm_device *dev)
>  		else
>  			kvm_arch_unregister_noncoherent_dma(dev->kvm);
>  	}
> -
> -	mutex_unlock(&kv->lock);
>  }
>  
>  static int kvm_vfio_group_add(struct kvm_device *dev, unsigned int fd)
> @@ -157,7 +153,7 @@ static int kvm_vfio_group_add(struct kvm_device *dev, unsigned int fd)
>  	/* Ensure the FD is a vfio group FD.*/
>  	if (!kvm_vfio_file_is_group(filp)) {
>  		ret = -EINVAL;
> -		goto err_fput;
> +		goto out_fput;
>  	}
>  
>  	mutex_lock(&kv->lock);
> @@ -165,30 +161,27 @@ static int kvm_vfio_group_add(struct kvm_device *dev, unsigned int fd)
>  	list_for_each_entry(kvg, &kv->group_list, node) {
>  		if (kvg->file == filp) {
>  			ret = -EEXIST;
> -			goto err_unlock;
> +			goto out_unlock;
>  		}
>  	}
>  
>  	kvg = kzalloc(sizeof(*kvg), GFP_KERNEL_ACCOUNT);
>  	if (!kvg) {
>  		ret = -ENOMEM;
> -		goto err_unlock;
> +		goto out_unlock;
>  	}
>  
> -	kvg->file = filp;
> +	kvg->file = get_file(filp);
>  	list_add_tail(&kvg->node, &kv->group_list);
>  
>  	kvm_arch_start_assignment(dev->kvm);
>  	kvm_vfio_file_set_kvm(kvg->file, dev->kvm);
> -
> -	mutex_unlock(&kv->lock);
> -
>  	kvm_vfio_update_coherency(dev);
>  
> -	return 0;
> -err_unlock:
> +	ret = 0;

Nit, let's initialize ret = 0 when it's declared to avoid this.  Series
looks good to me otherwise.  Thanks,

Alex

> +out_unlock:
>  	mutex_unlock(&kv->lock);
> -err_fput:
> +out_fput:
>  	fput(filp);
>  	return ret;
>  }
> @@ -224,12 +217,12 @@ static int kvm_vfio_group_del(struct kvm_device *dev, unsigned int fd)
>  		break;
>  	}
>  
> +	kvm_vfio_update_coherency(dev);
> +
>  	mutex_unlock(&kv->lock);
>  
>  	fdput(f);
>  
> -	kvm_vfio_update_coherency(dev);
> -
>  	return ret;
>  }
>