linux-kernel - Re: [PATCH v1 06/13] iommufd: Implement sw

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Message-ID: <20250211181620.GK3754072@nvidia.com>
Date: Tue, 11 Feb 2025 14:16:20 -0400
From: Jason Gunthorpe <jgg@...dia.com>
To: Nicolin Chen <nicolinc@...dia.com>
Cc: kevin.tian@...el.com, tglx@...utronix.de, maz@...nel.org,
	joro@...tes.org, will@...nel.org, robin.murphy@....com,
	shuah@...nel.org, iommu@...ts.linux.dev,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux-kselftest@...r.kernel.org, eric.auger@...hat.com,
	baolu.lu@...ux.intel.com, yi.l.liu@...el.com, yury.norov@...il.com,
	jacob.pan@...ux.microsoft.com, patches@...ts.linux.dev
Subject: Re: [PATCH v1 06/13] iommufd: Implement sw_msi support natively

On Sat, Feb 08, 2025 at 01:02:39AM -0800, Nicolin Chen wrote:

> +static struct iommufd_attach_handle *
> +iommu_group_get_iommufd_handle(struct iommu_group *group)
> +{
> +	struct iommu_attach_handle *handle;
> +
> +	handle = iommu_attach_handle_get(group, IOMMU_NO_PASID, 0);
> +	if (IS_ERR(handle))
> +		return NULL;
> +	return to_iommufd_handle(handle);
> +}
> +
> +/*
> + * Called by the irq code if the platform translates the MSI address through the
> + * IOMMU. msi_addr is the physical address of the MSI page. iommufd will
> + * allocate a fd global iova for the physical page that is the same on all
> + * domains and devices.
> + */
> +#ifdef CONFIG_IRQ_MSI_IOMMU
> +int iommufd_sw_msi(struct iommu_domain *domain, struct msi_desc *desc,
> +		   phys_addr_t msi_addr)
> +{
> +	struct device *dev = msi_desc_to_dev(desc);
> +	struct iommufd_hwpt_paging *hwpt_paging;
> +	struct iommufd_attach_handle *handle;
> +	struct iommufd_sw_msi_map *msi_map;
> +	struct iommufd_ctx *ictx;
> +	unsigned long iova;
> +	int rc;
> +
> +	handle = iommu_group_get_iommufd_handle(dev->iommu_group);
> +	if (!handle)
> +		return 0;

I think you should open code this and leave the other function
alone. The locking rules are different here.

iommufd_device_get_attach_handle() should be locked under the
igroup->lock

While in this context we are locked under the iommu core group mutex.

A comment will help

/*
 * It is safe to call iommu_attach_handle_get() here because the iommu
 * core code invokes this under the group mutex which also prevents any
 * change of the attach handle for the duration of this function.
 */
 iommu_group_mutex_assert(dev);

Jason