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Date: Wed, 21 Feb 2024 17:11:10 +0200
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Subject: Re: [PATCH 10/10] vfio/qat: Add vfio_pci driver for Intel QAT VF
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To: Jason Gunthorpe <jgg@nvidia.com>, "Zeng, Xin" <xin.zeng@intel.com>
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From: Yishai Hadas <yishaih@nvidia.com>
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On 21/02/2024 15:18, Jason Gunthorpe wrote:
> On Wed, Feb 21, 2024 at 08:44:31AM +0000, Zeng, Xin wrote:
>> On Wednesday, February 21, 2024 1:03 AM, Jason Gunthorpe wrote:
>>> On Tue, Feb 20, 2024 at 03:53:08PM +0000, Zeng, Xin wrote:
>>>> On Tuesday, February 20, 2024 9:25 PM, Jason Gunthorpe wrote:
>>>>> To: Zeng, Xin <xin.zeng@intel.com>
>>>>> Cc: Yishai Hadas <yishaih@nvidia.com>; herbert@gondor.apana.org.au;
>>>>> alex.williamson@redhat.com; shameerali.kolothum.thodi@huawei.com;
>>> Tian,
>>>>> Kevin <kevin.tian@intel.com>; linux-crypto@vger.kernel.org;
>>>>> kvm@vger.kernel.org; qat-linux <qat-linux@intel.com>; Cao, Yahui
>>>>> <yahui.cao@intel.com>
>>>>> Subject: Re: [PATCH 10/10] vfio/qat: Add vfio_pci driver for Intel QAT VF
>>> devices
>>>>>
>>>>> On Sat, Feb 17, 2024 at 04:20:20PM +0000, Zeng, Xin wrote:
>>>>>
>>>>>> Thanks for this information, but this flow is not clear to me why it
>>>>>> cause deadlock. From this flow, CPU0 is not waiting for any resource
>>>>>> held by CPU1, so after CPU0 releases mmap_lock, CPU1 can continue
>>>>>> to run. Am I missing something?
>>>>>
>>>>> At some point it was calling copy_to_user() under the state
>>>>> mutex. These days it doesn't.
>>>>>
>>>>> copy_to_user() would nest the mm_lock under the state mutex which is
>>> a
>>>>> locking inversion.
>>>>>
>>>>> So I wonder if we still have this problem now that the copy_to_user()
>>>>> is not under the mutex?
>>>>
>>>> In protocol v2, we still have the scenario in precopy_ioctl where
>>> copy_to_user is
>>>> called under state_mutex.
>>>
>>> Why? Does mlx5 do that? It looked Ok to me:
>>>
>>>          mlx5vf_state_mutex_unlock(mvdev);
>>>          if (copy_to_user((void __user *)arg, &info, minsz))
>>>                  return -EFAULT;
>>
>> Indeed, thanks, Jason. BTW, is there any reason why was "deferred_reset" mode
>> still implemented in mlx5 driver given this deadlock condition has been avoided
>> with migration protocol v2 implementation.
> 
> I do not remember. Yishai?
> 

Long time passed.., I also don't fully remember whether this was the 
only potential problem here, maybe Yes.

My plan is to prepare a cleanup patch for mlx5 and put it into our 
regression for a while, if all will be good, I may send it for the next 
kernel cycle.

By the way, there are other drivers around (i.e. hisi and mtty) that 
still run copy_to_user() under the state mutex and might hit the problem 
without the 'deferred_rest', see here[1].

If we'll reach to the conclusion that the only reason for that mechanism 
was the copy_to_user() under the state_mutex, those drivers can change 
their code easily and also send a patch to cleanup the 'deferred_reset'.

[1] 
https://elixir.bootlin.com/linux/v6.8-rc5/source/drivers/vfio/pci/hisilicon/hisi_acc_vfio_pci.c#L808
[2] 
https://elixir.bootlin.com/linux/v6.8-rc5/source/samples/vfio-mdev/mtty.c#L878

Yishai