Re: [PATCH v4 1/2] iommu/sva: Tighten SVA bind API with explicit flags

From: Jacob Pan <jacob.jun.pan@linux.intel.com>
To: "Luck, Tony" <tony.luck@intel.com>
Cc: "vkoul@kernel.org" <vkoul@kernel.org>,
	"Tian, Kevin" <kevin.tian@intel.com>,
	"Jiang, Dave" <dave.jiang@intel.com>,
	"Raj, Ashok" <ashok.raj@intel.com>,
	David Woodhouse <dwmw2@infradead.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Christoph Hellwig <hch@infradead.org>,
	"iommu@lists.linux-foundation.org"
	<iommu@lists.linux-foundation.org>,
	Jason Gunthorpe <jgg@nvidia.com>,
	"zhangfei.gao@linaro.org" <zhangfei.gao@linaro.org>,
	Jean-Philippe Brucker <jean-philippe@linaro.com>
Subject: Re: [PATCH v4 1/2] iommu/sva: Tighten SVA bind API with explicit flags
Date: Thu, 13 May 2021 13:22:51 -0700	[thread overview]
Message-ID: <20210513132251.0ff89b90@jacob-builder> (raw)
In-Reply-To: <20210513195749.GA801830@agluck-desk2.amr.corp.intel.com>

Hi Tony,

On Thu, 13 May 2021 12:57:49 -0700, "Luck, Tony" <tony.luck@intel.com>
wrote:

> On Thu, May 13, 2021 at 12:46:21PM -0700, Jacob Pan wrote:
> > It seems there are two options:
> > 1. Add a new IOMMU API to set up a system PASID with a *separate* IOMMU
> > page table/domain, mark the device is PASID only with a flag. Use DMA
> > APIs to explicit map/unmap. Based on this PASID-only flag, Vendor IOMMU
> > driver will decide whether to use system PASID domain during map/unmap.
> > Not clear if we also need to make IOVA==kernel VA.
> > 
> > 2. Add a new IOMMU API to setup a system PASID which points to
> > init_mm.pgd. This API only allows trusted device to bind with the
> > system PASID at its own risk. There is no need for DMA API. This is the
> > same as the current code except with an explicit API.
> > 
> > Which option?  
> 
> Option #1 looks cleaner to me. Option #2 gives access to bits
> of memory that the users of system PASID shouldn't ever need
> to touch ... just map regions of memory that the kernel has
> a "struct page" for.
> 
> What does "use DMA APIs to explicitly map/unmap" mean? Is that
> for the whole region?
> 
If we map the entire kernel direct map during system PASID setup, then we
don't need to use DMA API to map/unmap certain range.

I was thinking this system PASID page table could be on-demand. The mapping
is built by explicit use of DMA map/unmap APIs.

> I'm expecting that once this system PASID has been initialized,
> then any accelerator device with a kernel use case would use the
> same PASID. I.e. DSA for page clearing, IAX for ZSwap compression
> & decompression, etc.
> 
OK, sounds like we have to map the entire kernel VA with struct page as you
said. So we still by-pass DMA APIs, can we all agree on that?

> -Tony

Thanks,

Jacob
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