linux-kernel - Re: [PATCH v2 1/2] vfio: Allow No-IOMMU mode without checking iommu

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Message-ID: <20170720091909.GC17837@arm.com>
Date:   Thu, 20 Jul 2017 10:19:10 +0100
From:   Will Deacon <will.deacon@....com>
To:     Anup Patel <anup.patel@...adcom.com>
Cc:     Robin Murphy <robin.murphy@....com>,
        Joerg Roedel <joro@...tes.org>,
        Baptiste Reynal <b.reynal@...tualopensystems.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        Scott Branden <sbranden@...adcom.com>,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        iommu@...ts.linux-foundation.org, kvm@...r.kernel.org,
        bcm-kernel-feedback-list@...adcom.com
Subject: Re: [PATCH v2 1/2] vfio: Allow No-IOMMU mode without checking
 iommu_present()

On Thu, Jul 20, 2017 at 10:02:33AM +0530, Anup Patel wrote:
> Not allowing No-IOMMU mode for devices already having
> iommu_ops on their bus is very conservative.
> 
> We now have IOMMU (such as ARM SMMU) which can bypass
> transcations when IOMMU is not configured for a given
> device. In addition, it is not necessary to have all
> devices on bus to be upstream to an IOMMU on that bus.

How does the SMMU know to bypass in these cases? As I explained before, the
driver-specific command line option is the wrong way to go about arranging
this.

Will