linux-kernel - Re: [PATCH v2 1/6] iommu: Add iommu page fault cookie helpers

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Date:   Fri, 1 Dec 2023 10:38:10 -0400
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>,
        iommu@...ts.linux.dev, linux-kselftest@...r.kernel.org,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/6] iommu: Add iommu page fault cookie helpers

On Thu, Oct 26, 2023 at 10:49:25AM +0800, Lu Baolu wrote:

> +void *iopf_pasid_cookie_get(struct device *dev, ioasid_t pasid)
> +{
> +	struct iommu_fault_param *iopf_param = iopf_get_dev_fault_param(dev);
> +	void *curr;
> +
> +	if (!iopf_param)
> +		return ERR_PTR(-ENODEV);
> +
> +	xa_lock(&iopf_param->pasid_cookie);
> +	curr = xa_load(&iopf_param->pasid_cookie, pasid);
> +	xa_unlock(&iopf_param->pasid_cookie);

No need for this locking, the caller has to provide some kind of
locking to protect the returned pointer.

I'm not sure how this can work really..

What iommfd wants is to increment the device object refcount under
this xa_lock.

I'm not sure this is the right arrangement: Basically you want to
have a cookie per domain attachment for iopf domains that is forwarded
to the handler.

So maybe this entire thing is not quite right, instead of having a
generic iopf attached to the domain the iopf should be supplied at
domain attach time? Something like:

iommu_domain_attach_iopf(struct iommu_domain *, struct device *,
                         ioasid_t pasid, struct iopf *, void *cookie);

The per-attach cookie would be passed to the iopf function
automatically by the infrastructure.

Detach would have the necessary locking to ensure that no handler is
running across detach

Then the cookie is logically placed in the API and properly protected
with natural locking we already need.

Jason