linux-kernel - Re: [PATCH v2 1/5] iommu/vt-d: Separate page request queue from SVM

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Message-ID: <20240915134928.GD869260@ziepe.ca>
Date: Sun, 15 Sep 2024 10:49:28 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: Baolu Lu <baolu.lu@...ux.intel.com>
Cc: "Tian, Kevin" <kevin.tian@...el.com>,
	"j.granados@...sung.com" <j.granados@...sung.com>,
	David Woodhouse <dwmw2@...radead.org>,
	Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
	Robin Murphy <robin.murphy@....com>,
	Klaus Jensen <its@...elevant.dk>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"iommu@...ts.linux.dev" <iommu@...ts.linux.dev>
Subject: Re: [PATCH v2 1/5] iommu/vt-d: Separate page request queue from SVM

On Sat, Sep 14, 2024 at 01:49:44PM +0800, Baolu Lu wrote:
> On 2024/9/14 10:53, Tian, Kevin wrote:
> > > From: Baolu Lu<baolu.lu@...ux.intel.com>
> > > Sent: Saturday, September 14, 2024 9:18 AM
> > > 
> > > On 9/14/24 8:52 AM, Tian, Kevin wrote:
> > > > > From: Joel Granados via B4 Relay
> > > > > <devnull+j.granados.samsung.com@...nel.org>
> > > > > 
> > > > > From: Joel Granados<j.granados@...sung.com>
> > > > > 
> > > > > IO page faults are no longer dependent on CONFIG_INTEL_IOMMU_SVM.
> > > > > Move
> > > > > all Page Request Queue (PRQ) functions that handle prq events to a new
> > > > > file in drivers/iommu/intel/prq.c. The page_req_des struct is now
> > > > > declared in drivers/iommu/intel/prq.c.
> > > > > 
> > > > > No functional changes are intended. This is a preparation patch to
> > > > > enable the use of IO page faults outside the SVM/PASID use cases.
> > > > Do we want to guard it under a new config option e.g.
> > > > CONFIG_INTEL_IOMMU_IOPF? it's unnecessary to allocate resources
> > > > for the majority usages which don't require IOPF.
> > > > 
> > > > Baolu?
> > > The OS builder doesn't know if Linux will run on a platform with PRI-
> > > capable devices. They'll probably always enable this option if we
> > > provide it.
> > hmm then why do we need a SVM option? In reality I haven't seen
> > a platform which supports IOPF but no pasid/SVM. so the reason
> > for whether to have an option should be same between IOPF/SVM.
> > 
> > IMHO the point of options is to allow reducing footprint of the kernel
> > image and many options are probably always enabled in distributions...
> 
> To be honest, I would hope to remove the SVM option some day. It's
> nothing special except listening to an external notification and
> synchronize the caches when the page table is updated. It's common to
> all cases where a page table is shared between the IOMMU and another
> component.
> 
> As for CONFIG_INTEL_IOMMU_IOPF, my suggestion is that we don't need to
> add any unnecessary options unless we see a real need.

You could possibly bundle the SVA and IOPF options together

I called the new option on the ARM side CONFIG_ARM_SMMU_V3_IOMMUFD
which seems like a reasonable cut point against embedded vs server.

Jason