linux-kernel - Re: [PATCH v5 5/9] iommufd: Add iommufd fault object

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Date: Tue, 7 May 2024 21:11:21 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: Lu Baolu <baolu.lu@...ux.intel.com>
Cc: 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,
	virtualization@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 5/9] iommufd: Add iommufd fault object

On Tue, Apr 30, 2024 at 10:57:06PM +0800, Lu Baolu wrote:
> diff --git a/drivers/iommu/iommu-priv.h b/drivers/iommu/iommu-priv.h
> index ae65e0b85d69..1a0450a83bd0 100644
> --- a/drivers/iommu/iommu-priv.h
> +++ b/drivers/iommu/iommu-priv.h
> @@ -36,6 +36,10 @@ struct iommu_attach_handle {
>  			struct device	*dev;
>  			refcount_t	users;
>  		};
> +		/* attach data for IOMMUFD */
> +		struct {
> +			void		*idev;
> +		};

We can use a proper type here, just forward declare it.

But this sequence in the other patch:

+       ret = iommu_attach_group(hwpt->domain, idev->igroup->group);
+       if (ret) {
+               iommufd_fault_iopf_disable(idev);
+               return ret;
+       }
+
+       handle = iommu_attach_handle_get(idev->igroup->group, IOMMU_NO_PASID, 0);
+       handle->idev = idev;

Is why I was imagining the caller would allocate, because now we have
the issue that a fault capable domain was installed into the IOMMU
before it's handle could be fully setup, so we have a race where a
fault could come in right between those things. Then what happens?
I suppose we can retry the fault and by the time it comes back the
race should resolve. A bit ugly I suppose.

> diff --git a/include/uapi/linux/iommufd.h b/include/uapi/linux/iommufd.h
> index 83b45dce94a4..1819b28e9e6b 100644
> --- a/include/uapi/linux/iommufd.h
> +++ b/include/uapi/linux/iommufd.h
> @@ -50,6 +50,7 @@ enum {
>  	IOMMUFD_CMD_HWPT_SET_DIRTY_TRACKING,
>  	IOMMUFD_CMD_HWPT_GET_DIRTY_BITMAP,
>  	IOMMUFD_CMD_HWPT_INVALIDATE,
> +	IOMMUFD_CMD_FAULT_ALLOC,
>  };

I think I'd call this a CMD_FAULT_QUEUE_ALLOC - does that make sense?

Jason