linux-kernel - Re: [PATCH v4 2/9] iommu: Replace sva_iommu with iommu_attach

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Date: Wed, 3 Apr 2024 08:59:13 -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 v4 2/9] iommu: Replace sva_iommu with iommu_attach_handle

On Wed, Apr 03, 2024 at 09:15:12AM +0800, Lu Baolu wrote:
> +	/* A bond already exists, just take a reference`. */
> +	handle = iommu_attach_handle_get(group, iommu_mm->pasid);
> +	if (handle) {
> +		mutex_unlock(&iommu_sva_lock);
> +		return handle;
>  	}

At least in this context this is not enough we need to ensure that the
domain on the PASID is actually an SVA domain and it was installed by
this mechanism, not an iommufd domain for instance.

ie you probably need a type field in the iommu_attach_handle to tell
what the priv is.

Otherwise this seems like a great idea!

> -	iommu_detach_device_pasid(domain, dev, iommu_mm->pasid);
> -	if (--domain->users == 0) {
> -		list_del(&domain->next);
> -		iommu_domain_free(domain);
> +	iommu_attach_handle_put(handle);
> +	if (refcount_read(&handle->users) == 1) {
> +		iommu_detach_device_pasid(domain, dev, iommu_mm->pasid);
> +		if (--domain->users == 0) {
> +			list_del(&domain->next);
> +			iommu_domain_free(domain);
> +		}
>  	}

Though I'm not convinced the refcount should be elevated into the core
structure. The prior patch I showed you where the caller can provide
the memory for the handle and we don't have a priv would make it easy
to put the refcount in a SVA dervied handle struct without more
allocation. Then we don't need this weirdness.

>  	mutex_unlock(&iommu_sva_lock);
> -	kfree(handle);

Also do we need iommu_sva_lock here anymore? I wonder if the group
mutex would be sufficient..

Jason