linux-kernel - Re: [PATCH v5 6/7] iommu/vt-d: Implement set_dev

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Date:   Fri, 5 May 2023 13:32:02 -0700
From:   Jacob Pan <jacob.jun.pan@...ux.intel.com>
To:     Baolu Lu <baolu.lu@...ux.intel.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, iommu@...ts.linux.dev,
        Robin Murphy <robin.murphy@....com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Joerg Roedel <joro@...tes.org>, dmaengine@...r.kernel.org,
        vkoul@...nel.org, Will Deacon <will@...nel.org>,
        David Woodhouse <dwmw2@...radead.org>,
        Raj Ashok <ashok.raj@...el.com>,
        "Tian, Kevin" <kevin.tian@...el.com>, Yi Liu <yi.l.liu@...el.com>,
        "Yu, Fenghua" <fenghua.yu@...el.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        "Zanussi, Tom" <tom.zanussi@...el.com>,
        narayan.ranganathan@...el.com, jacob.jun.pan@...ux.intel.com
Subject: Re: [PATCH v5 6/7] iommu/vt-d: Implement set_dev_pasid domain op

Hi Baolu,

On Fri, 5 May 2023 10:58:38 +0800, Baolu Lu <baolu.lu@...ux.intel.com>
wrote:

> On 5/5/23 7:03 AM, Jacob Pan wrote:
> >>> +static int intel_iommu_attach_device_pasid(struct iommu_domain
> >>> *domain,
> >>> +					   struct device *dev,
> >>> ioasid_t pasid) +{
> >>> +	struct device_domain_info *info = dev_iommu_priv_get(dev);
> >>> +	struct dmar_domain *dmar_domain = to_dmar_domain(domain);
> >>> +	struct intel_iommu *iommu = info->iommu;
> >>> +	int ret;
> >>> +
> >>> +	if (!pasid_supported(iommu))
> >>> +		return -ENODEV;
> >>> +
> >>> +	ret = prepare_domain_attach_device(domain, dev);
> >>> +	if (ret)
> >>> +		return ret;
> >>> +
> >>> +	/*
> >>> +	 * Most likely the device context has already been set up,
> >>> will only
> >>> +	 * take a domain ID reference. Otherwise, device context will
> >>> be set
> >>> +	 * up here.  
> >> The "otherwise" case is only default domain deferred attaching case,
> >> right?  
> > it might be the only case so far, but my intention is to be general.
> > i.e. no ordering requirements. I believe it is more future proof in case
> > device_attach_pasid called before device_attach.  
> 
> Let's put aside deferred attach and talk about it later.
> 
> The device's context entry is configured when the default domain is
> being attached to the device. And, the default domain attaching is in
> the iommu probe_device path. It always happens before set_device_pasid
> which is designed to be called by the device driver. So the real
> situation is that when the device driver calls set_device_pasid, the
> context entry should already have been configured.
> 
> Then let's pick up the deferred attach case. It is a workaround for
> kdump (Documentation/admin-guide/kdump/kdump.rst). I don't think PASID
> feature is functionally required by any kdump capture kernel as its
> main purpose is to dump the memory of a panic kernel.
> 
> In summary, it seems to be reasonable for the vt-d driver to return
> -EBUSY when a device's context was copied. The idxd driver should
> continue to work without kernel DMA with PASID support.
> 
> 	if (context_copied(iommu, bus, devfn))
> 		return -EBUSY;
> 
> Make things general is always good, but this doesn't mean that we need
> to make the code complex to support cases that do not exist or are not
> used. Thoughts?
> 
Good point, it is better not put dead code in. Let me also document this
behavior for future change that may affect the ordering requirement.

Thanks,

Jacob