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

[Sathyanarayanan Kuppuswamy <sathyanarayanan.kuppuswamy@linux.intel.com>]

On 3/14/23 10:10 AM, Bjorn Helgaas wrote:
> On Tue, Mar 14, 2023 at 09:50:06AM -0700, Sathyanarayanan Kuppuswamy wrote:
>> On 3/14/23 9:02 AM, Bjorn Helgaas wrote:
>>> On Tue, Mar 14, 2023 at 08:06:07PM +0530, Ganapatrao Kulkarni wrote:
>>>> 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:
>>>>>>> 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.
>>>
>>>>>>>>> @@ -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.
>>>
>>> What would you think of removing the STU update feature from
>>> pci_enable_ats() so it always fails if pci_ats_stu_configure() has not
>>> been called, even when called on the PF, e.g.,
>>>
>>>   if (ps != pci_physfn(dev)->ats_stu)
>>>     return -EINVAL;
>>
>> If we are removing the STU update from pci_enable_ats(), why
>> even allow passing "ps (page shift)" parameter? IMO, we can assume that
>> for STU reconfigure, users will call pci_ats_stu_configure().
> 
> The reason to pass "ps" would be to verify that the STU the caller
> plans to use matches the actual STU.

Do we really need to verify it? My thinking is, by introducing
pci_ats_stu_configure() we are already trying to decouple the STU config
from pci_enable_ats(). So why again check for it when enabling ATS?

> 
>> Since zero is a valid STU, enabling ATS can be decoupled from STU
>> update.
>>
>>>   pci_read_config_word(dev, dev->ats_cap + PCI_ATS_CTRL, &ctrl);
>>>   ctrl |= PCI_ATS_CTRL_ENABLE;
>>>   pci_write_config_word(dev, dev->ats_cap + PCI_ATS_CTRL, ctrl);
>>>
>>> Would probably also have to set "dev->ats_stu = 0" in
>>> pci_disable_ats() to allow the possibility of calling
>>> pci_ats_stu_configure() again.
>>>
>>>> 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.
>>>
>>> Enabling or disabling ATS in a PF or VF has no effect on other
>>> functions.
>>>
>>> But changing STU while a VF has ATS enabled would definitely break any
>>> user of that VF, so if it's practical to verify that no VFs have ATS
>>> enabled, I think we should.
>>
>> I also think it is better to check for a ats_enabled status of VF before
>> configuring the STU.
>>
>> May be something like below (untested),
>>
>> static int is_ats_enabled_in_vf(struct pci_dev *dev)
>> {
>>         struct pci_sriov *iov = dev->sriov;
>>         struct pci_dev *vdev;
>>
>>         if (dev->is_virtfn) 
>>                 return -EINVAL;
>>
>>         for (i = 0; i < pci_sriov_get_totalvfs(pdev); i++) {
>>                 vdev = pci_get_domain_bus_and_slot(pci_domain_nr(dev->bus),
>>                                              pci_iov_virtfn_bus(dev, i),
>>                                              pci_iov_virtfn_devfn(dev, i));
> 
> I would try hard to avoid pci_get_domain_bus_and_slot().  That's
> expensive (searches *all* PCI devs with for_each_pci_dev()) and
> requires dealing with reference counts.
> 
> Maybe an atomic count in the PF of VFs with ATS enabled.
> 
>>                 if (vdev && vdev->ats_enabled)
>>                         return 1;
>>         }
>>
>>         return 0;
>>
>> }
>>
>> int pci_ats_stu_configure(struct pci_dev *dev, int ps)
>> {
>> ...
>> if (is_ats_enabled_in_vf(dev))
>>    return -EBUSY;
>>
>>>
>>>> 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; }
>>
>> -- 
>> Sathyanarayanan Kuppuswamy
>> Linux Kernel Developer
>>
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>> linux-arm-kernel mailing list
>> linux-arm-kernel@lists.infradead.org
>> http://lists.infradead.org/mailman/listinfo/linux-arm-kernel

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer