Re: [PATCH v2 1/2] PCI/ATS: Add a helper function to configure ATS STU of a PF

[Sathyanarayanan Kuppuswamy]

Hi Kulkarni,

On 3/13/23 2:12 PM, Bjorn Helgaas wrote:
> On Mon, Feb 27, 2023 at 08:21:36PM -0800, Ganapatrao Kulkarni wrote:
>> As per PCI specification (PCI Express Base Specification Revision
>> 6.0, Section 10.5) both PF and VFs of a PCI EP are permitted to be enabled
>> independently for ATS capability, however the STU(Smallest Translation
>> Unit) is shared between PF and VFs. For VFs, it is hardwired to Zero and
>> the associated PF's value applies to VFs.
>>
>> In the current code, the STU is being configured while enabling the PF ATS.
>> Hence, it is not able to enable ATS for VFs, if it is not enabled on the
>> associated PF already.
>>
>> Adding a function pci_ats_stu_configure(), which can be called to
>> configure the STU during PF enumeration.
>> Latter enumerations of VFs can successfully enable ATS independently.
> 
> s/STU(Smallest/STU (Smallest/ (add space before paren)
> s/Adding a function pci_ats_stu_configure()/Add pci_ats_stu_configure()/
> s/Latter/Subsequent/
> 
> Add blank line between paragraphs (it looks like "Latter enumerations"
> is intended to start a new paragraph).
> 
>> Signed-off-by: Ganapatrao Kulkarni <gankulkarni@xxxxxxxxxxxxxxxxxxxxxx>
> 
> Acked-by: Bjorn Helgaas <bhelgaas@xxxxxxxxxx>
> 
> Given an ack for the IOMMU patch, I'd be happy to merge both (and I
> can do the commit log tweaks); just let me know.
> 
> One comment/question below.
> 
>> ---
>>  drivers/pci/ats.c       | 33 +++++++++++++++++++++++++++++++--
>>  include/linux/pci-ats.h |  3 +++
>>  2 files changed, 34 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/pci/ats.c b/drivers/pci/ats.c
>> index f9cc2e10b676..1611bfa1d5da 100644
>> --- a/drivers/pci/ats.c
>> +++ b/drivers/pci/ats.c
>> @@ -46,6 +46,35 @@ bool pci_ats_supported(struct pci_dev *dev)
>>  }
>>  EXPORT_SYMBOL_GPL(pci_ats_supported);
>>  
>> +/**
>> + * pci_ats_stu_configure - Configure STU of a PF.
>> + * @dev: the PCI device
>> + * @ps: the IOMMU page shift
>> + *
>> + * Returns 0 on success, or negative on failure.
>> + */
>> +int pci_ats_stu_configure(struct pci_dev *dev, int ps)
>> +{
>> +	u16 ctrl;
>> +
>> +	if (dev->ats_enabled || dev->is_virtfn)
>> +		return 0;
> 
> I might return an error for the VF case on the assumption that it's
> likely an error in the caller.  I guess one could argue that it
> simplifies the caller if it doesn't have to check for PF vs VF.  But
> the fact that STU is shared between PF and VFs is an important part of
> understanding how ATS works, so the caller should be aware of the
> distinction anyway.

I have already asked this question. But let me repeat it.

We don't have any checks for the PF case here. That means you can re-configure
the STU as many times as you want until ATS is enabled in PF. So, if there are
active VFs which uses this STU, can PF re-configure the STU at will?

> 
>> +
>> +	if (!pci_ats_supported(dev))
>> +		return -EINVAL;
>> +
>> +	if (ps < PCI_ATS_MIN_STU)
>> +		return -EINVAL;
>> +
>> +	dev->ats_stu = ps;
>> +	pci_read_config_word(dev, dev->ats_cap + PCI_ATS_CTRL, &ctrl);
>> +	ctrl |= PCI_ATS_CTRL_STU(dev->ats_stu - PCI_ATS_MIN_STU);
>> +	pci_write_config_word(dev, dev->ats_cap + PCI_ATS_CTRL, ctrl);
>> +
>> +	return 0;
>> +}
>> +EXPORT_SYMBOL_GPL(pci_ats_stu_configure);
>> +
>>  /**
>>   * pci_enable_ats - enable the ATS capability
>>   * @dev: the PCI device
>> @@ -68,8 +97,8 @@ int pci_enable_ats(struct pci_dev *dev, int ps)
>>  		return -EINVAL;
>>  
>>  	/*
>> -	 * Note that enabling ATS on a VF fails unless it's already enabled
>> -	 * with the same STU on the PF.
>> +	 * Note that enabling ATS on a VF fails unless it's already
>> +	 * configured with the same STU on the PF.
>>  	 */
>>  	ctrl = PCI_ATS_CTRL_ENABLE;
>>  	if (dev->is_virtfn) {
>> diff --git a/include/linux/pci-ats.h b/include/linux/pci-ats.h
>> index df54cd5b15db..7d62a92aaf23 100644
>> --- a/include/linux/pci-ats.h
>> +++ b/include/linux/pci-ats.h
>> @@ -8,6 +8,7 @@
>>  /* Address Translation Service */
>>  bool pci_ats_supported(struct pci_dev *dev);
>>  int pci_enable_ats(struct pci_dev *dev, int ps);
>> +int pci_ats_stu_configure(struct pci_dev *dev, int ps);
>>  void pci_disable_ats(struct pci_dev *dev);
>>  int pci_ats_queue_depth(struct pci_dev *dev);
>>  int pci_ats_page_aligned(struct pci_dev *dev);
>> @@ -16,6 +17,8 @@ static inline bool pci_ats_supported(struct pci_dev *d)
>>  { return false; }
>>  static inline int pci_enable_ats(struct pci_dev *d, int ps)
>>  { return -ENODEV; }
>> +static inline int pci_ats_stu_configure(struct pci_dev *d, int ps)
>> +{ return -ENODEV; }
>>  static inline void pci_disable_ats(struct pci_dev *d) { }
>>  static inline int pci_ats_queue_depth(struct pci_dev *d)
>>  { return -ENODEV; }
>> -- 
>> 2.38.1
>>
>>
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-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer