linux-kernel - Re: [PATCH 4/4] dmaengine/idxd: Re-enable kernel workqueue under DMA API

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Date:   Fri, 3 Mar 2023 14:12:24 -0800
From:   Jacob Pan <jacob.jun.pan@...ux.intel.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
        Jason Gunthorpe <jgg@...dia.com>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        Joerg Roedel <joro@...tes.org>,
        "dmaengine@...r.kernel.org" <dmaengine@...r.kernel.org>,
        "vkoul@...nel.org" <vkoul@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        Will Deacon <will@...nel.org>,
        "David Woodhouse" <dwmw2@...radead.org>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "Liu, Yi L" <yi.l.liu@...el.com>,
        "Yu, Fenghua" <fenghua.yu@...el.com>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        "Luck, Tony" <tony.luck@...el.com>,
        "Zanussi, Tom" <tom.zanussi@...el.com>,
        jacob.jun.pan@...ux.intel.com
Subject: Re: [PATCH 4/4] dmaengine/idxd: Re-enable kernel workqueue under
 DMA API

Hi Kevin,

On Thu, 2 Mar 2023 09:47:00 +0000, "Tian, Kevin" <kevin.tian@...el.com>
wrote:

> > From: Jacob Pan <jacob.jun.pan@...ux.intel.com>
> > Sent: Thursday, March 2, 2023 9:00 AM
> > 
> >  static int idxd_enable_system_pasid(struct idxd_device *idxd)
> >  {
> > -	return -EOPNOTSUPP;
> > +	struct pci_dev *pdev = idxd->pdev;
> > +	struct device *dev = &pdev->dev;
> > +	struct iommu_domain *domain;
> > +	union gencfg_reg gencfg;
> > +	ioasid_t pasid;
> > +	int ret;
> > +
> > +	domain = iommu_get_domain_for_dev(dev);
> > +	if (!domain || domain->type == IOMMU_DOMAIN_BLOCKED)
> > +		return -EPERM;  
> 
> what about UNMANAGED?
will fix this by getting the dma domain.
> 
> > +
> > +	pasid = iommu_sva_reserve_pasid(1, dev->iommu->max_pasids);
> > +	if (pasid == IOMMU_PASID_INVALID)
> > +		return -ENOSPC;  
> 
> as commented in last patch we can just pass a device pointer to a
> general allocation interface.
will do
> 
> > +
> > +	ret = iommu_attach_device_pasid(domain, dev, pasid);
> > +	if (ret) {
> > +		dev_err(dev, "failed to attach device pasid %d, domain
> > type %d",
> > +			pasid, domain->type);
> > +		iommu_sva_unreserve_pasid(pasid);
> > +		return ret;
> > +	}
> > +
> > +	/* Since we set user privilege for kernel DMA, enable
> > completion IRQ */
> > +	gencfg.bits = ioread32(idxd->reg_base + IDXD_GENCFG_OFFSET);
> > +	gencfg.user_int_en = 1;
> > +	iowrite32(gencfg.bits, idxd->reg_base + IDXD_GENCFG_OFFSET);
> > +	idxd->pasid = pasid;  
> 
> Why does user privilege requires a completion interrupt?
> 
> Or instead it's more due to doing kernel DMA itself then we certainly
> don't want to poll in the kernel?
yes, kernel wq does not support polling, therefore it needs interrupts.
Without user_int_en bit set, there would be no interrupts if we use user
privilege for kernel wq.

Thanks,

Jacob