Re: [PATCH 1/4] ioasid: Reserve a global PASID for in-kernel DMA

From: Jacob Pan <jacob.jun.pan@linux.intel.com>
To: Jason Gunthorpe <jgg@nvidia.com>
Cc: Jean-Philippe Brucker <jean-philippe@linaro.org>,
	"Tian, Kevin" <kevin.tian@intel.com>,
	Tony Luck <tony.luck@intel.com>,
	Dave Jiang <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,
	Jacob Pan <jacob.jun.pan@intel.com>,
	Barry Song <21cnbao@gmail.com>,
	Dan Williams <dan.j.williams@intel.com>,
	Jean-Philippe Brucker <jean-philippe@linaro.com>,
	"Zanussi, Tom" <tom.zanussi@intel.com>
Subject: Re: [PATCH 1/4] ioasid: Reserve a global PASID for in-kernel DMA
Date: Fri, 10 Dec 2021 10:05:45 -0800	[thread overview]
Message-ID: <20211210100545.373c30d1@jacob-builder> (raw)
In-Reply-To: <20211210123109.GE6385@nvidia.com>

Hi Jason,

On Fri, 10 Dec 2021 08:31:09 -0400, Jason Gunthorpe <jgg@nvidia.com> wrote:

> On Fri, Dec 10, 2021 at 09:06:24AM +0000, Jean-Philippe Brucker wrote:
> > On Thu, Dec 09, 2021 at 10:14:04AM -0800, Jacob Pan wrote:  
> > > > This looks like we're just one step away from device drivers needing
> > > > multiple PASIDs for kernel DMA so I'm trying to figure out how to
> > > > evolve the API towards that. It's probably as simple as keeping a
> > > > kernel IOASID set at first, but then we'll probably want to
> > > > optimize by having multiple overlapping sets for each device driver
> > > > (all separate from the SVA set).  
> > > Sounds reasonable to start with a kernel set for in-kernel DMA once
> > > we need multiple ones. But I am not sure what *overlapping* sets mean
> > > here, could you explain?  
> > 
> > Given that each device uses a separate PASID table, we could allocate
> > the same set of PASID values for different device drivers. We just need
> > to make sure that those values are different from PASIDs allocated for
> > user SVA.  
> 
> Why does user SVA need global values anyhow?
> 
Currently, we have mm.pasid for user SVA. mm is global. We could have per
device PASID for dedicated devices (not shared across mm's), but that would
make things a lot more complex. I am thinking multiple PASIDs per mm is
needed, right?

For VT-d, the shared workqueue (SWQ) requires global PASIDs in that we
cannot have two processes use the same PASID to submit work on a workqueue
shared by the two processes. Each process's PASID must be unique to the
SWQ's PASID table.

> Jason

Thanks,

Jacob
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