linux-kernel - Re: [PATCH v3 4/8] iommufd: Add iommufd fault object

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Date: Mon, 25 Mar 2024 12:02:47 +0800
From: Baolu Lu <baolu.lu@...ux.intel.com>
To: Jason Gunthorpe <jgg@...pe.ca>,
 Shameerali Kolothum Thodi <shameerali.kolothum.thodi@...wei.com>
Cc: baolu.lu@...ux.intel.com, Kevin Tian <kevin.tian@...el.com>,
 Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
 Robin Murphy <robin.murphy@....com>,
 Jean-Philippe Brucker <jean-philippe@...aro.org>,
 Nicolin Chen <nicolinc@...dia.com>, Yi Liu <yi.l.liu@...el.com>,
 Jacob Pan <jacob.jun.pan@...ux.intel.com>,
 Joel Granados <j.granados@...sung.com>,
 "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
 "virtualization@...ts.linux-foundation.org"
 <virtualization@...ts.linux-foundation.org>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 4/8] iommufd: Add iommufd fault object

On 3/25/24 11:26 AM, Baolu Lu wrote:
> On 3/23/24 1:22 AM, Jason Gunthorpe wrote:
>> On Wed, Mar 20, 2024 at 04:18:05PM +0000, Shameerali Kolothum Thodi 
>> wrote:
>>> What I have noticed is that,
>>> -read interface works fine and I can receive struct 
>>> tiommu_hwpt_pgfault data.
>>> -But once Guest handles the page faults and returns the page response,
>>>   the write to fault fd never reaches the kernel. The sequence is 
>>> like below,
>>>    sqe = io_uring_get_sqe(ring);
>>>    io_uring_prep_write(sqe, hwpt->fault_fd, resp, sizeof(*resp), 0);
>>>    io_uring_sqe_set_data(sqe, resp);
>>>    io_uring_submit(ring);
>>>    ret = io_uring_wait_cqe(ring, &cqe);
>>>    ....
>>> Please find the function here[2]
>>>
>>> The above cqe wait never returns and hardware times out without 
>>> receiving
>>> page response. My understanding of io_uring default op is that it 
>>> tries to
>>> issue an sqe as non-blocking first. But it looks like the above write 
>>> sequence
>>> ends up in kernel poll_wait() as well.Not sure how we can avoid that for
>>> write.
>> Ah, right, it is because poll can't be choosy about read/write, it has
>> to work equally for both directions. iommufd_fault_fops_poll() never
>> returns EPOLLOUT
>>
>> It should just always return EPOLLOUT because we don't have any queue
>> to manage.
> 
> Are you suggesting the poll file operation to be like below?
> 
> static __poll_t iommufd_fault_fops_poll(struct file *filep,
>                                          struct poll_table_struct *wait)
> {
>          struct iommufd_fault *fault = filep->private_data;
>          __poll_t pollflags = EPOLLOUT;
> 
>          poll_wait(filep, &fault->wait_queue, wait);
>          mutex_lock(&fault->mutex);
>          if (!list_empty(&fault->deliver))
>                  pollflags = EPOLLIN | EPOLLRDNORM;

If I understood it correctly, here it should be,

		pollflags |= EPOLLIN | EPOLLRDNORM;

>          mutex_unlock(&fault->mutex);
> 
>          return pollflags;
> }

Best regards,
baolu