Re: [PATCH 4/4] dmaengine: idxd: Use DMA API for in-kernel DMA with PASID

From: Jacob Pan <jacob.jun.pan@linux.intel.com>
To: "Tian, Kevin" <kevin.tian@intel.com>
Cc: "Luck, Tony" <tony.luck@intel.com>,
	"Jiang, Dave" <dave.jiang@intel.com>,
	"Raj, Ashok" <ashok.raj@intel.com>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	"Kumar, Sanjay K" <sanjay.k.kumar@intel.com>,
	LKML <linux-kernel@vger.kernel.org>,
	Christoph Hellwig <hch@infradead.org>,
	"iommu@lists.linux-foundation.org"
	<iommu@lists.linux-foundation.org>,
	"Pan, Jacob jun" <jacob.jun.pan@intel.com>,
	Jason Gunthorpe <jgg@nvidia.com>, Barry Song <21cnbao@gmail.com>,
	"Williams, Dan J" <dan.j.williams@intel.com>,
	Jean-Philippe Brucker <jean-philippe@linaro.com>,
	"Zanussi, Tom" <tom.zanussi@intel.com>
Subject: Re: [PATCH 4/4] dmaengine: idxd: Use DMA API for in-kernel DMA with PASID
Date: Thu, 9 Dec 2021 11:18:17 -0800	[thread overview]
Message-ID: <20211209111817.6a84a55c@jacob-builder> (raw)
In-Reply-To: <BN9PR11MB527617F8CC015E4F15EDCC9F8C709@BN9PR11MB5276.namprd11.prod.outlook.com>

Hi Kevin,

On Thu, 9 Dec 2021 01:48:09 +0000, "Tian, Kevin" <kevin.tian@intel.com>
wrote:

> > From: Jason Gunthorpe <jgg@nvidia.com>
> > Sent: Thursday, December 9, 2021 1:51 AM
> >   
> > > > > +	/*
> > > > > +	 * Try to enable both in-kernel and user DMA request
> > > > > with PASID.
> > > > > +	 * PASID is supported unless both user and kernel PASID
> > > > > are
> > > > > +	 * supported. Do not fail probe here in that idxd can
> > > > > still be
> > > > > +	 * used w/o PASID or IOMMU.
> > > > > +	 */
> > > > > +	if (iommu_dev_enable_feature(dev, IOMMU_DEV_FEAT_SVA) ||
> > > > > +		idxd_enable_system_pasid(idxd)) {
> > > > > +		dev_warn(dev, "Failed to enable PASID\n");
> > > > > +	} else {
> > > > > +		set_bit(IDXD_FLAG_PASID_ENABLED, &idxd->flags);
> > > > >   	}  
> > > > Huh? How can the driver keep going if PASID isn't supported? I
> > > > thought the whole point of this was because the device cannot do
> > > > DMA without PASID at all?  
> > >
> > > There are 2 types of WQ supported with the DSA devices. A dedicated
> > > WQ  
> > type  
> > > and a shared WQ type. The dedicated WQ type can support DMA with and  
> > without  
> > > PASID. The shared wq type must have a PASID to operate. The driver can
> > > support dedicated WQ only without PASID usage when there is no PASID
> > > support.  
> > 
> > Can you add to the cover letter why does the kernel require to use the
> > shared WQ?
> > 
> > Jason  
> 
> Two reasons:
> 
> On native the shared WQ is useful when the kernel wants to offload
> some memory operations (e.g. page-zeroing) to DSA. When #CPUs are
> more than #WQs, this allows per-cpu lock-less submissions using
> ENQCMD(PASID, payload) instruction.
> 
> In guest the virtual DSA HW may only contain a WQ in shared mode
> (unchangeable by the guest) when the host admin wants to share
> the limited WQ resource among many VMs. Then there is no choice
> in guest regardless whether it's for user or kernel controlled DMA.
I will add these to the next cover letter.

Thanks,

Jacob
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