Re: Remapping BOs and registers post BAR movements [was - Re: Question about supporting AMD eGPU hot plug case]

[Andrey Grodzovsky <andrey.grodzovsky@amd.com>]

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Andrey

On 2021-03-02 12:48 p.m., Andrey Grodzovsky wrote:
> Adding PCI mailing list per Bjorn's advise (sorry, skipped the address 
> last time).
> 
> On 2021-03-02 12:22 p.m., Christian König wrote:
>> Hi Andrey,
>>
>>> Can you elaborate more why you need such a lock on a global level and 
>>> not
>>> enough per driver (per device more specifically) local lock acquired in
>>> rescan_preapare (before BARs shuffling starts) and released in 
>>> rescan_done
>>> (after BARs shuffling done) ? 
>>
>> because this adds inter device dependencies.
>>
>> E.g. when we have device A, B and C and each is taking it's own lock 
>> we end up with 3 locks.
>>
>> If those device have inter dependencies on their own, for example 
>> because of DMA-buf we will end up in quite a locking mess.
>>
>> Apart from that if we don't always keep the order A, B, C but instead 
>> end up with A, C, B lockdep will certainly start to complain.
>>
>> Regards,
>> Christian.
> 
> But as far as I understand those locks are agnostic to each other - each 
> lock is taken only by it's respective device and not by others. So I 
> don't see deadlock danger here but, I do suspect lockdep might throw a 
> false warning...
> 
> Andrey
> 
>>
>> Am 02.03.21 um 18:16 schrieb Andrey Grodzovsky:
>>> Per Bjorn's advise adding PCI mailing list.
>>>
>>> Christian, please reply on top of this mail and not on top the 
>>> earlier, original one.
>>>
>>> Andrey
>>>
>>> On 2021-03-02 11:49 a.m., Bjorn Helgaas wrote:
>>>> Please add linux-pci@vger.kernel.org to the cc list.  There's a lot of
>>>> useful context in this thread and it needs to be archived.
>>>> On Tue, Mar 02, 2021 at 11:37:25AM -0500, Andrey Grodzovsky wrote:
>>>>> Adding Sergei and Bjorn since you are suggesting some PCI related 
>>>>> changes.
>>>>>
>>>>> On 2021-03-02 6:28 a.m., Christian König wrote:
>>>>>> Yeah, I'm thinking about that as well for quite a while now.
>>>>>>
>>>>>> I'm not 100% sure what Sergei does in his patch set will work for 
>>>>>> us. He
>>>>>> basically tells the driver to stop any processing then shuffle around
>>>>>> the BARs and then tells him to start again.
>>>>>>
>>>>>> Because of the necessity to reprogram bridges there isn't much 
>>>>>> other way
>>>>>> how to do this, but we would need something like taking a lock,
>>>>>> returning and dropping the lock later on. If you do this for multiple
>>>>>> drivers at the same time this results in a locking nightmare.
>>>>>>
>>>>>> What we need instead is a upper level PCI layer rw-lock which we can
>>>>>> take on the read side in the driver, similar to our internal reset 
>>>>>> lock.
>>>>>
>>>>>
>>>>> Can you elaborate more why you need such a lock on a global level 
>>>>> and not
>>>>> enough per driver (per device more specifically) local lock 
>>>>> acquired in
>>>>> rescan_preapare (before BARs shuffling starts) and released in 
>>>>> rescan_done
>>>>> (after BARs shuffling done) ?
>>>>>
>>>>> Andrey
>>>>>
>>>>>
>>>>>>
>>>>>> Wiring up changing the mapping is the smaller problem in that 
>>>>>> picture.
>>>>>>
>>>>>> Christian.
>>>>>>
>>>>>> Am 01.03.21 um 22:14 schrieb Andrey Grodzovsky:
>>>>>>> Hi, I started looking into actual implementation for this, as
>>>>>>> reference point I am using Sege's NVME implementations (see 
>>>>>>> https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2FYADRO-KNS%2Flinux%2Fcommit%2F58c375df086502538706ee23e8ef7c8bb5a4178f&data=04%7C01%7Candrey.grodzovsky%40amd.com%7C44bc68a68feb4b71836c08d8dd9b306a%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637503005898302093%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=nuTptAjGewG2T7tXOQr2h%2Bdr42lyg3m1%2Bfv0mJzd8tw%3D&reserved=0) 
>>>>>>>
>>>>>>> and our own amdgpu_device_resize_fb_bar. Our use case being more
>>>>>>> complicated then NVME's arises some questions or thoughts:
>>>>>>>
>>>>>>> Before the PCI core will move the BARs (VRAM and registers) we have
>>>>>>> to stop the HW and SW and then unmap all MMIO mapped entitles in the
>>>>>>> driver. This includes, registers of all types and  VRAM placed BOs
>>>>>>> with CPU visibility. My concern is how to prevent accesses to all
>>>>>>> MMIO ranges between iounmap in rescan_preapare and ioremap in
>>>>>>> rescan_done. For register accesses we are again back to same issue
>>>>>>> we had with device unplug and GPU reset we are discussing with Denis
>>>>>>> now. So here at least as first solution just to confirm the feature
>>>>>>> works I can introduce low level (in register accessors) RW locking
>>>>>>> to avoid access until all register access driver pointers are
>>>>>>> remapped post BARs move. What more concerns me is how to - In
>>>>>>> rescan_preapare: collect all CPU accessible BOs placed in VRAM (i
>>>>>>> can use amdgpu specific list like I currently use for device
>>>>>>> unplug), lock them to prevent any further use of them, unpin them
>>>>>>> (and unmap). In rescan_preapare: remap them back and unlock them for
>>>>>>> further use. Also, there is a difference between kernel BOs where
>>>>>>> indeed iounmap, ioremap should be enough and user space mapped BOs
>>>>>>> where it seems like in hot unplug, we need in rescan_preapare to
>>>>>>> only unamp them, in between drop new page faults with VM_FAULT_RETRY
>>>>>>> and post rescan_done allow page faults to go through to refill all
>>>>>>> the user page tables with updated MMIO addresses.
>>>>>>>
>>>>>>> Let me know your thoughts.
>>>>>>>
>>>>>>> Andrey
>>>>>>>
>>>>>>> On 2021-02-26 4:03 p.m., Andrey Grodzovsky wrote:
>>>>>>>>
>>>>>>>> On 2021-02-26 1:44 a.m., Sergei Miroshnichenko wrote:
>>>>>>>>> On Thu, 2021-02-25 at 13:28 -0500, Andrey Grodzovsky wrote:
>>>>>>>>>> On 2021-02-25 2:00 a.m., Sergei Miroshnichenko wrote:
>>>>>>>>>>> On Wed, 2021-02-24 at 17:51 -0500, Andrey Grodzovsky wrote:
>>>>>>>>>>>> On 2021-02-24 1:23 p.m., Sergei Miroshnichenko wrote:
>>>>>>>>>>>>> ...
>>>>>>>>>>>> Are you saying that even without hot-plugging, while both nvme
>>>>>>>>>>>> and
>>>>>>>>>>>> AMD
>>>>>>>>>>>> card are present
>>>>>>>>>>>> right from boot, you still get BARs moving and MMIO ranges
>>>>>>>>>>>> reassigned
>>>>>>>>>>>> for NVME BARs
>>>>>>>>>>>> just because amdgpu driver will start resize of AMD card 
>>>>>>>>>>>> BARs and
>>>>>>>>>>>> this
>>>>>>>>>>>> will trigger NVMEs BARs move to
>>>>>>>>>>>> allow AMD card BARs to cover full range of VIDEO RAM ?
>>>>>>>>>>> Yes. Unconditionally, because it is unknown beforehand if NVMe's
>>>>>>>>>>> BAR
>>>>>>>>>>> movement will help. In this particular case BAR movement is not
>>>>>>>>>>> needed,
>>>>>>>>>>> but is done anyway.
>>>>>>>>>>>
>>>>>>>>>>> BARs are not moved one by one, but the kernel releases all the
>>>>>>>>>>> releasable ones, and then recalculates a new BAR layout to 
>>>>>>>>>>> fit them
>>>>>>>>>>> all. Kernel's algorithm is different from BIOS's, so NVME has
>>>>>>>>>>> appeared
>>>>>>>>>>> at a new place.
>>>>>>>>>>>
>>>>>>>>>>> This is triggered by following:
>>>>>>>>>>> - at boot, if BIOS had assigned not every BAR;
>>>>>>>>>>> - during pci_resize_resource();
>>>>>>>>>>> - during pci_rescan_bus() -- after a pciehp event or a manual 
>>>>>>>>>>> via
>>>>>>>>>>> sysfs.
>>>>>>>>>> By manual via sysfs you mean something like this - 'echo 1 >
>>>>>>>>>> /sys/bus/pci/drivers/amdgpu/0000\:0c\:00.0/remove && echo 1 >
>>>>>>>>>> /sys/bus/pci/rescan ' ? I am looking into how most reliably 
>>>>>>>>>> trigger
>>>>>>>>>> PCI
>>>>>>>>>> code to call my callbacks even without having external PCI 
>>>>>>>>>> cage for
>>>>>>>>>> GPU
>>>>>>>>>> (will take me some time to get it).
>>>>>>>>> Yeah, this is our way to go when a device can't be physically 
>>>>>>>>> removed
>>>>>>>>> or unpowered remotely. With just a bit shorter path:
>>>>>>>>>
>>>>>>>>>     sudo sh -c 'echo 1 > 
>>>>>>>>> /sys/bus/pci/devices/0000\:0c\:00.0/remove'
>>>>>>>>>     sudo sh -c 'echo 1 > /sys/bus/pci/rescan'
>>>>>>>>>
>>>>>>>>> Or, just a second command (rescan) is enough: a BAR movement 
>>>>>>>>> attempt
>>>>>>>>> will be triggered even if there were no changes in PCI topology.
>>>>>>>>>
>>>>>>>>> Serge
>>>>>>>>
>>>>>>>>
>>>>>>>> Ok, able to trigger stub callbacks call after doing rescan from
>>>>>>>> sysfs. Also I see BARs movement
>>>>>>>>
>>>>>>>> [ 1860.968036] amdgpu 0000:09:00.0: BAR 0: assigned [mem
>>>>>>>> 0xc0000000-0xcfffffff 64bit pref]
>>>>>>>> [ 1860.968050] amdgpu 0000:09:00.0: BAR 0 updated: 0xe0000000 ->
>>>>>>>> 0xc0000000
>>>>>>>> [ 1860.968054] amdgpu 0000:09:00.0: BAR 2: assigned [mem
>>>>>>>> 0xd0000000-0xd01fffff 64bit pref]
>>>>>>>> [ 1860.968068] amdgpu 0000:09:00.0: BAR 2 updated: 0xf0000000 ->
>>>>>>>> 0xd0000000
>>>>>>>> [ 1860.968071] amdgpu 0000:09:00.0: BAR 5: assigned [mem
>>>>>>>> 0xf0100000-0xf013ffff]
>>>>>>>> [ 1860.968078] amdgpu 0000:09:00.0: BAR 5 updated: 0xfce00000 ->
>>>>>>>> 0xf0100000
>>>>>>>>
>>>>>>>> As expected, after the move the device becomes unresponsive as
>>>>>>>> unmap/ioremap wasn't done.
>>>>>>>>
>>>>>>>> Andrey
>>>>>>>>
>>>>>>
>>