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Subject: Re: [PATCH] vfio/pci: Parallelize device open and release
To:     Alex Williamson <alex.williamson@redhat.com>
Cc:     kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
        christian.ehrhardt@canonical.com
References: <154180134334.31154.16289123769826098135.stgit@gimli.home>
From:   Auger Eric <eric.auger@redhat.com>
Message-ID: <48822457-863a-77fc-01b9-f9eca7190435@redhat.com>
Date:   Tue, 13 Nov 2018 15:42:49 +0100
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Hi Alex,

On 11/9/18 11:09 PM, Alex Williamson wrote:
> In commit 61d792562b53 ("vfio-pci: Use mutex around open, release, and
> remove") a mutex was added to freeze the refcnt for a device so that
> we can handle errors and perform bus resets on final close.  However,
> bus resets can be rather slow and a global mutex here is undesirable.
> A per-device mutex provides the best granularity, but then our chances
> of triggering a bus/slot reset with multiple affected devices is slim
> when devices are released in parallel.
Sorry I don't get the above sentence.
  Instead create a reflck object
> shared among all devices under the same bus or slot, allowing devices
> on independent buses to be released in parallel while serializing per
> bus/slot.>
> Reported-by: Christian Ehrhardt <christian.ehrhardt@canonical.com>
> Tested-by: Christian Ehrhardt <christian.ehrhardt@canonical.com>
> Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
> ---
>  drivers/vfio/pci/vfio_pci.c         |  157 ++++++++++++++++++++++++++++++-----
>  drivers/vfio/pci/vfio_pci_private.h |    6 +
>  2 files changed, 139 insertions(+), 24 deletions(-)
> 
> diff --git a/drivers/vfio/pci/vfio_pci.c b/drivers/vfio/pci/vfio_pci.c
> index 50cdedfca9fe..d443fb7a4e75 100644
> --- a/drivers/vfio/pci/vfio_pci.c
> +++ b/drivers/vfio/pci/vfio_pci.c
> @@ -56,8 +56,6 @@ module_param(disable_idle_d3, bool, S_IRUGO | S_IWUSR);
>  MODULE_PARM_DESC(disable_idle_d3,
>  		 "Disable using the PCI D3 low power state for idle, unused devices");
>  
> -static DEFINE_MUTEX(driver_lock);
> -
>  static inline bool vfio_vga_disabled(void)
>  {
>  #ifdef CONFIG_VFIO_PCI_VGA
> @@ -393,14 +391,14 @@ static void vfio_pci_release(void *device_data)
>  {
>  	struct vfio_pci_device *vdev = device_data;
>  
> -	mutex_lock(&driver_lock);
> +	mutex_lock(&vdev->reflck->lock);
>  
>  	if (!(--vdev->refcnt)) {
>  		vfio_spapr_pci_eeh_release(vdev->pdev);
>  		vfio_pci_disable(vdev);
>  	}
>  
> -	mutex_unlock(&driver_lock);
> +	mutex_unlock(&vdev->reflck->lock);
>  
>  	module_put(THIS_MODULE);
>  }
> @@ -413,7 +411,7 @@ static int vfio_pci_open(void *device_data)
>  	if (!try_module_get(THIS_MODULE))
>  		return -ENODEV;
>  
> -	mutex_lock(&driver_lock);
> +	mutex_lock(&vdev->reflck->lock);
>  
>  	if (!vdev->refcnt) {
>  		ret = vfio_pci_enable(vdev);
> @@ -424,7 +422,7 @@ static int vfio_pci_open(void *device_data)
>  	}
>  	vdev->refcnt++;
>  error:
> -	mutex_unlock(&driver_lock);
> +	mutex_unlock(&vdev->reflck->lock);
>  	if (ret)
>  		module_put(THIS_MODULE);
>  	return ret;
> @@ -1187,6 +1185,9 @@ static const struct vfio_device_ops vfio_pci_ops = {
>  	.request	= vfio_pci_request,
>  };
>  
> +static int vfio_pci_reflck_attach(struct vfio_pci_device *vdev);
> +static void vfio_pci_reflck_put(struct vfio_pci_reflck *reflck);
> +
>  static int vfio_pci_probe(struct pci_dev *pdev, const struct pci_device_id *id)
>  {
>  	struct vfio_pci_device *vdev;
> @@ -1233,6 +1234,14 @@ static int vfio_pci_probe(struct pci_dev *pdev, const struct pci_device_id *id)
>  		return ret;
>  	}
>  
> +	ret = vfio_pci_reflck_attach(vdev);
> +	if (ret) {
> +		vfio_del_group_dev(&pdev->dev);
> +		vfio_iommu_group_put(group, &pdev->dev);
> +		kfree(vdev);
> +		return ret;
> +	}
> +
>  	if (vfio_pci_is_vga(pdev)) {
>  		vga_client_register(pdev, vdev, NULL, vfio_pci_set_vga_decode);
>  		vga_set_legacy_decoding(pdev,
> @@ -1264,6 +1273,8 @@ static void vfio_pci_remove(struct pci_dev *pdev)
>  	if (!vdev)
>  		return;
>  
> +	vfio_pci_reflck_put(vdev->reflck);
> +
>  	vfio_iommu_group_put(pdev->dev.iommu_group, &pdev->dev);
>  	kfree(vdev->region);
>  	mutex_destroy(&vdev->ioeventfds_lock);
> @@ -1320,16 +1331,97 @@ static struct pci_driver vfio_pci_driver = {
>  	.err_handler	= &vfio_err_handlers,
>  };
>  
> +static DEFINE_MUTEX(reflck_lock);
> +
> +static struct vfio_pci_reflck *vfio_pci_reflck_alloc(void)
> +{
> +	struct vfio_pci_reflck *reflck;
> +
> +	reflck = kzalloc(sizeof(*reflck), GFP_KERNEL);
> +	if (!reflck)
> +		return ERR_PTR(-ENOMEM);
> +
> +	kref_init(&reflck->kref);
> +	mutex_init(&reflck->lock);
> +
> +	return reflck;
> +}
> +
> +static void vfio_pci_reflck_get(struct vfio_pci_reflck *reflck)
> +{
> +	kref_get(&reflck->kref);
> +}
> +
> +static int vfio_pci_reflck_find(struct pci_dev *pdev, void *data)
> +{
> +	struct vfio_pci_reflck **preflck = data;
> +	struct vfio_device *device;
> +	struct vfio_pci_device *vdev;
> +
> +	device = vfio_device_get_from_dev(&pdev->dev);
> +	if (!device)
> +		return 0;
> +
> +	if (pci_dev_driver(pdev) != &vfio_pci_driver) {
> +		vfio_device_put(device);
> +		return 0;
> +	}
> +
> +	vdev = vfio_device_data(device);
> +
> +	if (vdev->reflck) {
> +		vfio_pci_reflck_get(vdev->reflck);
> +		*preflck = vdev->reflck;
> +		vfio_device_put(device);
> +		return 1;
> +	}
> +
> +	vfio_device_put(device);
> +	return 0;
> +}
> +
> +static int vfio_pci_reflck_attach(struct vfio_pci_device *vdev)
> +{
> +	bool slot = !pci_probe_reset_slot(vdev->pdev->slot);
> +
> +	mutex_lock(&reflck_lock);
> +
> +	if (pci_is_root_bus(vdev->pdev->bus) ||
> +	    vfio_pci_for_each_slot_or_bus(vdev->pdev, vfio_pci_reflck_find,
> +					  &vdev->reflck, slot) <= 0)
!vfio_pci_for_each_slot_or_bus(vdev->pdev, vfio_pci_reflck_find,
					  &vdev->reflck, slot)?
I don't think we can have negative returned value.
> +		vdev->reflck = vfio_pci_reflck_alloc();
> +
> +	mutex_unlock(&reflck_lock);
> +
> +	return IS_ERR(vdev->reflck) ? PTR_ERR(vdev->reflck) : 0;
> +}
> +
> +static void vfio_pci_reflck_release(struct kref *kref)
> +{
> +	struct vfio_pci_reflck *reflck = container_of(kref,
> +						      struct vfio_pci_reflck,
> +						      kref);
> +
> +	kfree(reflck);
> +	mutex_unlock(&reflck_lock);
> +}
> +
> +static void vfio_pci_reflck_put(struct vfio_pci_reflck *reflck)
> +{
> +	kref_put_mutex(&reflck->kref, vfio_pci_reflck_release, &reflck_lock);
> +}
> +
>  struct vfio_devices {
>  	struct vfio_device **devices;
>  	int cur_index;
>  	int max_index;
>  };
>  
> -static int vfio_pci_get_devs(struct pci_dev *pdev, void *data)
> +static int vfio_pci_get_unused_devs(struct pci_dev *pdev, void *data)
>  {
>  	struct vfio_devices *devs = data;
>  	struct vfio_device *device;
> +	struct vfio_pci_device *vdev;
>  
>  	if (devs->cur_index == devs->max_index)
>  		return -ENOSPC;
> @@ -1343,16 +1435,25 @@ static int vfio_pci_get_devs(struct pci_dev *pdev, void *data)
>  		return -EBUSY;
>  	}
>  
> +	vdev = vfio_device_data(device);
> +
> +	/* Only collect unused devices */
abort if the slot/bus features a used device?
> +	if (vdev->refcnt) {
> +		vfio_device_put(device);
> +		return -EBUSY;
> +	}
> +
>  	devs->devices[devs->cur_index++] = device;
>  	return 0;
>  }
>  
>  /*
> - * Attempt to do a bus/slot reset if there are devices affected by a reset for
> - * this device that are needs_reset and all of the affected devices are unused
> - * (!refcnt). Callers are required to hold driver_lock when calling this to
> - * prevent device opens and concurrent bus reset attempts.  We prevent device
> - * unbinds by acquiring and holding a reference to the vfio_device.
> + * Attempt to do a bus/slot reset if there are devices affected by a reset
> + * for this device that are "needs_reset" and all of the affected devices
> + * are unused (!refcnt).

This comment still sounds a bit cryptic to me. Assuming I got the point,
may I suggest something like:

If one of the device of the slot/bus needs a reset (but cannot be reset
at function level) and all the devices of the slot/bus are unused
(!refcount), we attempt to do a bus/slot reset.

  Callers are required to hold vdev->reflck->lock
> + * to prevent concurrent device opens, which is expected to protect all
> + * affected devices.  A vfio_device reference is also acquired for each
> + * affected device to prevent unbinds.
>   *
>   * NB: vfio-core considers a group to be viable even if some devices are
>   * bound to drivers like pci-stub or pcieport.  Here we require all devices
> @@ -1363,7 +1464,7 @@ static void vfio_pci_try_bus_reset(struct vfio_pci_device *vdev)
>  {
>  	struct vfio_devices devs = { .cur_index = 0 };
>  	int i = 0, ret = -EINVAL;
> -	bool needs_reset = false, slot = false;
> +	bool slot = false;
>  	struct vfio_pci_device *tmp;
>  
>  	if (!pci_probe_reset_slot(vdev->pdev->slot))
> @@ -1381,28 +1482,36 @@ static void vfio_pci_try_bus_reset(struct vfio_pci_device *vdev)
>  		return;
>  
>  	if (vfio_pci_for_each_slot_or_bus(vdev->pdev,
> -					  vfio_pci_get_devs, &devs, slot))
> +					  vfio_pci_get_unused_devs,
> +					  &devs, slot))
>  		goto put_devs;
>  
> +	/* Does at least one need a reset? */
>  	for (i = 0; i < devs.cur_index; i++) {
>  		tmp = vfio_device_data(devs.devices[i]);
> -		if (tmp->needs_reset)
> -			needs_reset = true;
> -		if (tmp->refcnt)
> -			goto put_devs;
> +		if (tmp->needs_reset) {
> +			ret = pci_reset_bus(vdev->pdev);
> +			break;
> +		}
>  	}
>  
> -	if (needs_reset)
> -		ret = pci_reset_bus(vdev->pdev);
> -
>  put_devs:
>  	for (i = 0; i < devs.cur_index; i++) {
>  		tmp = vfio_device_data(devs.devices[i]);
> -		if (!ret)
> +
> +		/*
> +		 * If reset was successful, affected devices no longer need
> +		 * a reset and we should return all the collateral devices
> +		 * to low power.  If not successful, we either didn't reset
> +		 * the bus or timed out waiting for it, so let's not touch
> +		 * the power state.
> +		 */
> +		if (!ret) {
>  			tmp->needs_reset = false;
>  
> -		if (!tmp->refcnt && !disable_idle_d3)
> -			pci_set_power_state(tmp->pdev, PCI_D3hot);
> +			if (tmp != vdev && !disable_idle_d3)
> +				pci_set_power_state(tmp->pdev, PCI_D3hot);
> +		}
>  
>  		vfio_device_put(devs.devices[i]);
>  	}
> diff --git a/drivers/vfio/pci/vfio_pci_private.h b/drivers/vfio/pci/vfio_pci_private.h
> index cde3b5d3441a..aa2355e67340 100644
> --- a/drivers/vfio/pci/vfio_pci_private.h
> +++ b/drivers/vfio/pci/vfio_pci_private.h
> @@ -76,6 +76,11 @@ struct vfio_pci_dummy_resource {
>  	struct list_head	res_next;
>  };
>  
> +struct vfio_pci_reflck {
> +	struct kref		kref;
> +	struct mutex		lock;
> +};
> +
>  struct vfio_pci_device {
>  	struct pci_dev		*pdev;
>  	void __iomem		*barmap[PCI_STD_RESOURCE_END + 1];
> @@ -104,6 +109,7 @@ struct vfio_pci_device {
>  	bool			needs_reset;
>  	bool			nointx;
>  	struct pci_saved_state	*pci_saved_state;
> +	struct vfio_pci_reflck	*reflck;
>  	int			refcnt;
>  	int			ioeventfds_nr;
>  	struct eventfd_ctx	*err_trigger;
> 
Reviewed-by: Eric Auger <eric.auger@redhat.com>

Thanks

Eric