Re: [PATCH v5 4/7] PCI/ATS: Add PRI support for PCIe VF devices

[Bjorn Helgaas <helgaas@kernel.org>]

[+cc Joerg, David, iommu list: because IOMMU drivers are the only
callers of pci_enable_pri() and pci_enable_pasid()]

On Thu, Aug 01, 2019 at 05:06:01PM -0700, sathyanarayanan.kuppuswamy@linux.intel.com wrote:
> From: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
> 
> When IOMMU tries to enable Page Request Interface (PRI) for VF device
> in iommu_enable_dev_iotlb(), it always fails because PRI support for
> PCIe VF device is currently broken. Current implementation expects
> the given PCIe device (PF & VF) to implement PRI capability before
> enabling the PRI support. But this assumption is incorrect. As per PCIe
> spec r4.0, sec 9.3.7.11, all VFs associated with PF can only use the
> PRI of the PF and not implement it. Hence we need to create exception
> for handling the PRI support for PCIe VF device.
> 
> Also, since PRI is a shared resource between PF/VF, following rules
> should apply.
> 
> 1. Use proper locking before accessing/modifying PF resources in VF
>    PRI enable/disable call.
> 2. Use reference count logic to track the usage of PRI resource.
> 3. Disable PRI only if the PRI reference count (pri_ref_cnt) is zero.

Wait, why do we need this at all?  I agree the spec says VFs may not
implement PRI or PASID capabilities and that VFs use the PRI and
PASID of the PF.

But why do we need to support pci_enable_pri() and pci_enable_pasid()
for VFs?  There's nothing interesting we can *do* in the VF, and
passing it off to the PF adds all this locking mess.  For VFs, can we
just make them do nothing or return -EINVAL?  What functionality would
we be missing if we did that?

(Obviously returning -EINVAL would require tweaks in the callers to
either avoid the call for VFs or handle the -EINVAL gracefully.)

> Cc: Ashok Raj <ashok.raj@intel.com>
> Cc: Keith Busch <keith.busch@intel.com>
> Suggested-by: Ashok Raj <ashok.raj@intel.com>
> Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
> ---
>  drivers/pci/ats.c   | 143 ++++++++++++++++++++++++++++++++++----------
>  include/linux/pci.h |   2 +
>  2 files changed, 112 insertions(+), 33 deletions(-)
> 
> diff --git a/drivers/pci/ats.c b/drivers/pci/ats.c
> index 1f4be27a071d..079dc5444444 100644
> --- a/drivers/pci/ats.c
> +++ b/drivers/pci/ats.c
> @@ -189,6 +189,8 @@ void pci_pri_init(struct pci_dev *pdev)
>  	if (pdev->is_virtfn)
>  		return;
>  
> +	mutex_init(&pdev->pri_lock);
> +
>  	pos = pci_find_ext_capability(pdev, PCI_EXT_CAP_ID_PRI);
>  	if (!pos)
>  		return;
> @@ -221,29 +223,57 @@ int pci_enable_pri(struct pci_dev *pdev, u32 reqs)
>  {
>  	u16 control, status;
>  	u32 max_requests;
> +	int ret = 0;
> +	struct pci_dev *pf = pci_physfn(pdev);
>  
> -	if (WARN_ON(pdev->pri_enabled))
> -		return -EBUSY;
> +	mutex_lock(&pf->pri_lock);
>  
> -	if (!pdev->pri_cap)
> -		return -EINVAL;
> +	if (WARN_ON(pdev->pri_enabled)) {
> +		ret = -EBUSY;
> +		goto pri_unlock;
> +	}
>  
> -	pci_read_config_word(pdev, pdev->pri_cap + PCI_PRI_STATUS, &status);
> -	if (!(status & PCI_PRI_STATUS_STOPPED))
> -		return -EBUSY;
> +	if (!pf->pri_cap) {
> +		ret = -EINVAL;
> +		goto pri_unlock;
> +	}
> +
> +	if (pdev->is_virtfn && pf->pri_enabled)
> +		goto update_status;
> +
> +	/*
> +	 * Before updating PRI registers, make sure there is no
> +	 * outstanding PRI requests.
> +	 */
> +	pci_read_config_word(pf, pf->pri_cap + PCI_PRI_STATUS, &status);
> +	if (!(status & PCI_PRI_STATUS_STOPPED)) {
> +		ret = -EBUSY;
> +		goto pri_unlock;
> +	}
>  
> -	pci_read_config_dword(pdev, pdev->pri_cap + PCI_PRI_MAX_REQ,
> -			      &max_requests);
> +	pci_read_config_dword(pf, pf->pri_cap + PCI_PRI_MAX_REQ, &max_requests);
>  	reqs = min(max_requests, reqs);
> -	pdev->pri_reqs_alloc = reqs;
> -	pci_write_config_dword(pdev, pdev->pri_cap + PCI_PRI_ALLOC_REQ, reqs);
> +	pf->pri_reqs_alloc = reqs;
> +	pci_write_config_dword(pf, pf->pri_cap + PCI_PRI_ALLOC_REQ, reqs);
>  
>  	control = PCI_PRI_CTRL_ENABLE;
> -	pci_write_config_word(pdev, pdev->pri_cap + PCI_PRI_CTRL, control);
> +	pci_write_config_word(pf, pf->pri_cap + PCI_PRI_CTRL, control);
>  
> -	pdev->pri_enabled = 1;
> +	/*
> +	 * If PRI is not already enabled in PF, increment the PF
> +	 * pri_ref_cnt to track the usage of PRI interface.
> +	 */
> +	if (pdev->is_virtfn && !pf->pri_enabled) {
> +		atomic_inc(&pf->pri_ref_cnt);
> +		pf->pri_enabled = 1;
> +	}
>  
> -	return 0;
> +update_status:
> +	atomic_inc(&pf->pri_ref_cnt);
> +	pdev->pri_enabled = 1;
> +pri_unlock:
> +	mutex_unlock(&pf->pri_lock);
> +	return ret;
>  }
>  EXPORT_SYMBOL_GPL(pci_enable_pri);
>  
> @@ -256,18 +286,30 @@ EXPORT_SYMBOL_GPL(pci_enable_pri);
>  void pci_disable_pri(struct pci_dev *pdev)
>  {
>  	u16 control;
> +	struct pci_dev *pf = pci_physfn(pdev);
>  
> -	if (WARN_ON(!pdev->pri_enabled))
> -		return;
> +	mutex_lock(&pf->pri_lock);
>  
> -	if (!pdev->pri_cap)
> -		return;
> +	if (WARN_ON(!pdev->pri_enabled) || !pf->pri_cap)
> +		goto pri_unlock;
> +
> +	atomic_dec(&pf->pri_ref_cnt);
>  
> -	pci_read_config_word(pdev, pdev->pri_cap + PCI_PRI_CTRL, &control);
> +	/*
> +	 * If pri_ref_cnt is not zero, then don't modify hardware
> +	 * registers.
> +	 */
> +	if (atomic_read(&pf->pri_ref_cnt))
> +		goto done;
> +
> +	pci_read_config_word(pf, pf->pri_cap + PCI_PRI_CTRL, &control);
>  	control &= ~PCI_PRI_CTRL_ENABLE;
> -	pci_write_config_word(pdev, pdev->pri_cap + PCI_PRI_CTRL, control);
> +	pci_write_config_word(pf, pf->pri_cap + PCI_PRI_CTRL, control);
>  
> +done:
>  	pdev->pri_enabled = 0;
> +pri_unlock:
> +	mutex_unlock(&pf->pri_lock);
>  }
>  EXPORT_SYMBOL_GPL(pci_disable_pri);
>  
> @@ -277,17 +319,31 @@ EXPORT_SYMBOL_GPL(pci_disable_pri);
>   */
>  void pci_restore_pri_state(struct pci_dev *pdev)
>  {
> -	u16 control = PCI_PRI_CTRL_ENABLE;
> -	u32 reqs = pdev->pri_reqs_alloc;
> +	u16 control;
> +	u32 reqs;
> +	struct pci_dev *pf = pci_physfn(pdev);
>  
>  	if (!pdev->pri_enabled)
>  		return;
>  
> -	if (!pdev->pri_cap)
> +	if (!pf->pri_cap)
>  		return;
>  
> -	pci_write_config_dword(pdev, pdev->pri_cap + PCI_PRI_ALLOC_REQ, reqs);
> -	pci_write_config_word(pdev, pdev->pri_cap + PCI_PRI_CTRL, control);
> +	mutex_lock(&pf->pri_lock);
> +
> +	/* If PRI is already enabled by other VF's or PF, return */
> +	pci_read_config_word(pf, pf->pri_cap + PCI_PRI_CTRL, &control);
> +	if (control & PCI_PRI_CTRL_ENABLE)
> +		goto pri_unlock;
> +
> +	reqs = pf->pri_reqs_alloc;
> +	control = PCI_PRI_CTRL_ENABLE;
> +
> +	pci_write_config_dword(pf, pf->pri_cap + PCI_PRI_ALLOC_REQ, reqs);
> +	pci_write_config_word(pf, pf->pri_cap + PCI_PRI_CTRL, control);
> +
> +pri_unlock:
> +	mutex_unlock(&pf->pri_lock);
>  }
>  EXPORT_SYMBOL_GPL(pci_restore_pri_state);
>  
> @@ -300,18 +356,32 @@ EXPORT_SYMBOL_GPL(pci_restore_pri_state);
>   */
>  int pci_reset_pri(struct pci_dev *pdev)
>  {
> +	struct pci_dev *pf = pci_physfn(pdev);
>  	u16 control;
> +	int ret = 0;
>  
> -	if (WARN_ON(pdev->pri_enabled))
> -		return -EBUSY;
> +	mutex_lock(&pf->pri_lock);
>  
> -	if (!pdev->pri_cap)
> -		return -EINVAL;
> +	if (WARN_ON(pdev->pri_enabled)) {
> +		ret = -EBUSY;
> +		goto done;
> +	}
> +
> +	if (!pf->pri_cap) {
> +		ret = -EINVAL;
> +		goto done;
> +	}
> +
> +	/* If PRI is already enabled by other VF's or PF, return 0 */
> +	if (pf->pri_enabled)
> +		goto done;
>  
>  	control = PCI_PRI_CTRL_RESET;
> -	pci_write_config_word(pdev, pdev->pri_cap + PCI_PRI_CTRL, control);
>  
> -	return 0;
> +	pci_write_config_word(pf, pf->pri_cap + PCI_PRI_CTRL, control);
> +done:
> +	mutex_unlock(&pf->pri_lock);
> +	return ret;
>  }
>  EXPORT_SYMBOL_GPL(pci_reset_pri);
>  #endif /* CONFIG_PCI_PRI */
> @@ -475,11 +545,18 @@ EXPORT_SYMBOL_GPL(pci_pasid_features);
>  int pci_prg_resp_pasid_required(struct pci_dev *pdev)
>  {
>  	u16 status;
> +	struct pci_dev *pf = pci_physfn(pdev);
> +
> +	mutex_lock(&pf->pri_lock);
>  
> -	if (!pdev->pri_cap)
> +	if (!pf->pri_cap) {
> +		mutex_unlock(&pf->pri_lock);
>  		return 0;
> +	}
> +
> +	pci_read_config_word(pf, pf->pri_cap + PCI_PRI_STATUS, &status);
>  
> -	pci_read_config_word(pdev, pdev->pri_cap + PCI_PRI_STATUS, &status);
> +	mutex_unlock(&pf->pri_lock);
>  
>  	if (status & PCI_PRI_STATUS_PASID)
>  		return 1;
> diff --git a/include/linux/pci.h b/include/linux/pci.h
> index 27224c0db849..3c9c4c82be27 100644
> --- a/include/linux/pci.h
> +++ b/include/linux/pci.h
> @@ -455,8 +455,10 @@ struct pci_dev {
>  	atomic_t	ats_ref_cnt;	/* Number of VFs with ATS enabled */
>  #endif
>  #ifdef CONFIG_PCI_PRI
> +	struct mutex	pri_lock;	/* PRI enable lock */
>  	u16		pri_cap;	/* PRI Capability offset */
>  	u32		pri_reqs_alloc; /* Number of PRI requests allocated */
> +	atomic_t	pri_ref_cnt;	/* Number of PF/VF PRI users */
>  #endif
>  #ifdef CONFIG_PCI_PASID
>  	u16		pasid_cap;	/* PASID Capability offset */
> -- 
> 2.21.0
>