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

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Message-ID: <e6504514-9e95-4f68-a276-35582db3881e@linux.intel.com>
Date:   Fri, 8 Dec 2023 14:24:19 +0800
From:   Baolu Lu <baolu.lu@...ux.intel.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     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>,
        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 12/1/23 10:38 PM, Jason Gunthorpe wrote:
> 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.

Great idea! In a subsequent series, we could arrange the enabling and
disabling of IOPF in this API, thereby eliminating the calling of
iommu_dev_enable/disable_feature(dev, IOMMU_DEV_FEAT_IOPF) from the
device drivers.

Best regards,
baolu