Date:   Tue, 14 Mar 2023 20:06:07 +0530
From:   Ganapatrao Kulkarni <gankulkarni@...amperecomputing.com>
To:     Sathyanarayanan Kuppuswamy 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Bjorn Helgaas <helgaas@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, iommu@...ts.linux.dev,
        joro@...tes.org, bhelgaas@...gle.com, robin.murphy@....com,
        will@...nel.org, jean-philippe@...aro.org,
        darren@...amperecomputing.com, scott@...amperecomputing.com
Subject: Re: [PATCH v2 1/2] PCI/ATS: Add a helper function to configure ATS
 STU of a PF


Hi Sathyanarayanan,

On 14-03-2023 06:22 pm, Sathyanarayanan Kuppuswamy wrote:
> 
> 
> On 3/14/23 3:08 AM, Ganapatrao Kulkarni wrote:
>>
>>
>> On 14-03-2023 04:00 am, Sathyanarayanan Kuppuswamy wrote:
>>> 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@...amperecomputing.com>
>>>>
>>>> Acked-by: Bjorn Helgaas <bhelgaas@...gle.com>
>>>>
>>>> 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?
>>>
>>
>> IMO, Since STU is shared, programming it multiple times is not expected from callers code do it, however we can add below check to allow to program STU once from a PF.
>>
>> diff --git a/drivers/pci/ats.c b/drivers/pci/ats.c
>> index 1611bfa1d5da..f7bb01068e18 100644
>> --- a/drivers/pci/ats.c
>> +++ b/drivers/pci/ats.c
>> @@ -60,6 +60,10 @@ int pci_ats_stu_configure(struct pci_dev *dev, int ps)
>>          if (dev->ats_enabled || dev->is_virtfn)
>>                  return 0;
>>
>> +       /* Configured already */
>> +       if (dev->ats_stu)
>> +               return 0;
>> +
> 
> Theoretically, you can re-configure STU as long as no one is using it. Instead of this check, is
> there a way to check whether there are active VMs which enables ATS?

Yes I agree, there is no limitation on how many times you write STU 
bits, but practically it is happening while PF is enumerated.

The usage of function pci_ats_stu_configure is almost similar(subset) to
pci_enable_ats and only difference is one does ATS enable + STU program 
and another does only STU program.

IMO, I dont think, there is any need to find how many active VMs with 
attached VFs and it is not done for pci_enable_ats as well.

Also the caller has the requirement to call either pci_ats_stu_configure 
or pci_enable_ats while enumerating the PF.

> 
>>          if (!pci_ats_supported(dev))
>>                  return -EINVAL;
>>>>
>>>>> +
>>>>> +    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
>>>>>
>>>>>
>>>>> _______________________________________________
>>>>> linux-arm-kernel mailing list
>>>>> linux-arm-kernel@...ts.infradead.org
>>>>> http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
>>>
>>
>> Thanks,
>> Ganapat
>>
> 
Thanks,
Ganapat

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