Re: [PATCH v3 1/3] PCI: Introduce pcibios_ignore_alignment_request

[Alexey Kardashevskiy <aik@ozlabs.ru>]

On 31/05/2019 08:49, Shawn Anastasio wrote:
> On 5/29/19 10:39 PM, Alexey Kardashevskiy wrote:
>>
>>
>> On 28/05/2019 17:39, Shawn Anastasio wrote:
>>>
>>>
>>> On 5/28/19 1:27 AM, Alexey Kardashevskiy wrote:
>>>>
>>>>
>>>> On 28/05/2019 15:36, Oliver wrote:
>>>>> On Tue, May 28, 2019 at 2:03 PM Shawn Anastasio <shawn@anastas.io>
>>>>> wrote:
>>>>>>
>>>>>> Introduce a new pcibios function pcibios_ignore_alignment_request
>>>>>> which allows the PCI core to defer to platform-specific code to
>>>>>> determine whether or not to ignore alignment requests for PCI
>>>>>> resources.
>>>>>>
>>>>>> The existing behavior is to simply ignore alignment requests when
>>>>>> PCI_PROBE_ONLY is set. This is behavior is maintained by the
>>>>>> default implementation of pcibios_ignore_alignment_request.
>>>>>>
>>>>>> Signed-off-by: Shawn Anastasio <shawn@anastas.io>
>>>>>> ---
>>>>>>    drivers/pci/pci.c   | 9 +++++++--
>>>>>>    include/linux/pci.h | 1 +
>>>>>>    2 files changed, 8 insertions(+), 2 deletions(-)
>>>>>>
>>>>>> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
>>>>>> index 8abc843b1615..8207a09085d1 100644
>>>>>> --- a/drivers/pci/pci.c
>>>>>> +++ b/drivers/pci/pci.c
>>>>>> @@ -5882,6 +5882,11 @@ resource_size_t __weak
>>>>>> pcibios_default_alignment(void)
>>>>>>           return 0;
>>>>>>    }
>>>>>>
>>>>>> +int __weak pcibios_ignore_alignment_request(void)
>>>>>> +{
>>>>>> +       return pci_has_flag(PCI_PROBE_ONLY);
>>>>>> +}
>>>>>> +
>>>>>>    #define RESOURCE_ALIGNMENT_PARAM_SIZE COMMAND_LINE_SIZE
>>>>>>    static char
>>>>>> resource_alignment_param[RESOURCE_ALIGNMENT_PARAM_SIZE] = {0};
>>>>>>    static DEFINE_SPINLOCK(resource_alignment_lock);
>>>>>> @@ -5906,9 +5911,9 @@ static resource_size_t
>>>>>> pci_specified_resource_alignment(struct pci_dev *dev,
>>>>>>           p = resource_alignment_param;
>>>>>>           if (!*p && !align)
>>>>>>                   goto out;
>>>>>> -       if (pci_has_flag(PCI_PROBE_ONLY)) {
>>>>>> +       if (pcibios_ignore_alignment_request()) {
>>>>>>                   align = 0;
>>>>>> -               pr_info_once("PCI: Ignoring requested alignments
>>>>>> (PCI_PROBE_ONLY)\n");
>>>>>> +               pr_info_once("PCI: Ignoring requested alignments\n");
>>>>>>                   goto out;
>>>>>>           }
>>>>>
>>>>> I think the logic here is questionable to begin with. If the user has
>>>>> explicitly requested re-aligning a resource via the command line then
>>>>> we should probably do it even if PCI_PROBE_ONLY is set. When it breaks
>>>>> they get to keep the pieces.
>>>>>
>>>>> That said, the real issue here is that PCI_PROBE_ONLY probably
>>>>> shouldn't be set under qemu/kvm. Under the other hypervisor (PowerVM)
>>>>> hotplugged devices are configured by firmware before it's passed to
>>>>> the guest and we need to keep the FW assignments otherwise things
>>>>> break. QEMU however doesn't do any BAR assignments and relies on that
>>>>> being handled by the guest. At boot time this is done by SLOF, but
>>>>> Linux only keeps SLOF around until it's extracted the device-tree.
>>>>> Once that's done SLOF gets blown away and the kernel needs to do it's
>>>>> own BAR assignments. I'm guessing there's a hack in there to make it
>>>>> work today, but it's a little surprising that it works at all...
>>>>
>>>>
>>>> The hack is to run a modified qemu-aware "/usr/sbin/rtas_errd" in the
>>>> guest which receives an event from qemu (RAS_EPOW from
>>>> /proc/interrupts), fetches device tree chunks (and as I understand it -
>>>> they come with BARs from phyp but without from qemu) and writes "1" to
>>>> "/sys/bus/pci/rescan" which calls pci_assign_resource() eventually:
>>>
>>> Interesting. Does this mean that the PHYP hotplug path doesn't
>>> call pci_assign_resource?
>>
>>
>> I'd expect dlpar_add_slot() to be called under phyp and eventually
>> pci_device_add() which (I think) may or may not trigger later
>> reassignment.
>>
>>
>>> If so it means the patch may not
>>> break that platform after all, though it still may not be
>>> the correct way of doing things.
>>
>>
>> We should probably stop enforcing the PCI_PROBE_ONLY flag - it seems
>> that (unless resource_alignment= is used) the pseries guest should just
>> walk through all allocated resources and leave them unchanged.
> 
> If we add a pcibios_default_alignment() implementation like was
> suggested earlier, then it will behave as if the user has
> specified resource_alignment= by default and SLOF's assignments
> won't be honored (I think).


I removed pci_add_flags(PCI_PROBE_ONLY) from pSeries_setup_arch and
tried booting with and without pci=resource_alignment= and I can see no
difference - BARs are still aligned to 64K as programmed in SLOF; if I
hack SLOF to align to 4K or 32K - BARs get packed and the guest leaves
them unchanged.


> I guess it boils down to one question - is it important that we
> observe SLOF's initial BAR assignments?

It isn't if it's SLOF but it is if it's phyp. It used to not
allow/support BAR reassignment and even if it does not, I'd rather avoid
touching them.


> If not, the device tree
> modification that Sam found would work fine here. Otherwise,
> we need a way to honor the initial assignments from SLOF while
> still allowing custom alignments for hotplugged devices, either
> by deferring to the platform code like I do here, unsetting
> PCI_PROBE_ONLY in certain cases or by using IORESOURCE_PCI_FIXED
> like Bjorn suggested.
> 
>>
>>
>>>> [c000000006e6f960] [c0000000005f62d4] pci_assign_resource+0x44/0x360
>>>>
>>>> [c000000006e6fa10] [c0000000005f8b54]
>>>> assign_requested_resources_sorted+0x84/0x110
>>>> [c000000006e6fa60] [c0000000005f9540]
>>>> __assign_resources_sorted+0xd0/0x750
>>>> [c000000006e6fb40] [c0000000005fb2e0]
>>>> __pci_bus_assign_resources+0x80/0x280
>>>> [c000000006e6fc00] [c0000000005fb95c]
>>>> pci_assign_unassigned_bus_resources+0xbc/0x100
>>>> [c000000006e6fc60] [c0000000005e3d74] pci_rescan_bus+0x34/0x60
>>>>
>>>> [c000000006e6fc90] [c0000000005f1ef4] rescan_store+0x84/0xc0
>>>>
>>>> [c000000006e6fcd0] [c00000000068060c] bus_attr_store+0x3c/0x60
>>>>
>>>> [c000000006e6fcf0] [c00000000037853c] sysfs_kf_write+0x5c/0x80
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>>
>>>>> IIRC Sam Bobroff was looking at hotplug under pseries recently so he
>>>>> might have something to add. He's sick at the moment, but I'll ask him
>>>>> to take a look at this once he's back among the living
>>>>>
>>>>>> diff --git a/include/linux/pci.h b/include/linux/pci.h
>>>>>> index 4a5a84d7bdd4..47471dcdbaf9 100644
>>>>>> --- a/include/linux/pci.h
>>>>>> +++ b/include/linux/pci.h
>>>>>> @@ -1990,6 +1990,7 @@ static inline void
>>>>>> pcibios_penalize_isa_irq(int irq, int active) {}
>>>>>>    int pcibios_alloc_irq(struct pci_dev *dev);
>>>>>>    void pcibios_free_irq(struct pci_dev *dev);
>>>>>>    resource_size_t pcibios_default_alignment(void);
>>>>>> +int pcibios_ignore_alignment_request(void);
>>>>>>
>>>>>>    #ifdef CONFIG_HIBERNATE_CALLBACKS
>>>>>>    extern struct dev_pm_ops pcibios_pm_ops;
>>>>>> -- 
>>>>>> 2.20.1
>>>>>>
>>>>
>>

-- 
Alexey