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

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Message-ID: <20240322172232.GK66976@ziepe.ca>
Date: Fri, 22 Mar 2024 14:22:32 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: Shameerali Kolothum Thodi <shameerali.kolothum.thodi@...wei.com>
Cc: Lu Baolu <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 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.

Jason