linux-kernel - Re: [PATCH 4/5] iommu/vt-d: Add support for static identity domain

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Date:   Wed, 29 Nov 2023 16:28:51 -0400
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Lu Baolu <baolu.lu@...ux.intel.com>
Cc:     Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        Kevin Tian <kevin.tian@...el.com>, iommu@...ts.linux.dev,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/5] iommu/vt-d: Add support for static identity domain

On Wed, Nov 29, 2023 at 04:26:15PM -0400, Jason Gunthorpe wrote:
> On Mon, Nov 20, 2023 at 07:29:43PM +0800, Lu Baolu wrote:
> 
> > @@ -2311,6 +2316,13 @@ static int device_def_domain_type(struct device *dev)
> >  			return IOMMU_DOMAIN_IDENTITY;
> >  	}
> >  
> > +	/*
> > +	 * Hardware does not support the passthrough translation mode.
> > +	 * Always use a dynamaic mapping domain.
> > +	 */
> > +	if (!ecap_pass_through(iommu->ecap))
> > +		return IOMMU_DOMAIN_DMA;
> > +
> 
> Doesn't this return from def_domain_type completely prevent using an
> identity domain?
> 
> I thought the point of this was to allow the identity domain but have
> it be translating?

I suppose the answer is the next patch deletes that stuff.

I would probably have structured this in the other order, first add
this hunk and say that old HW is being de-supported. Remove all the
now-dead code creating the 1:1 page table, then refactor the remainder
to create the global static.

Jason