linux-kernel - Re: [RFC 0/2] arm-smmu-v3 tlbi-on-map option

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Date:   Wed, 12 Jul 2017 18:54:56 +0100
From:   Will Deacon <will.deacon@....com>
To:     Eric Auger <eric.auger@...hat.com>
Cc:     eric.auger.pro@...il.com, iommu@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, robin.murphy@....com,
        Jean-Philippe.Brucker@....com, christoffer.dall@...aro.org,
        Marc.Zyngier@....com, alex.williamson@...hat.com,
        peterx@...hat.com, mst@...hat.com
Subject: Re: [RFC 0/2] arm-smmu-v3 tlbi-on-map option

Hi Eric,

On Sun, Jul 09, 2017 at 05:15:01PM +0200, Eric Auger wrote:
> This series adds a new tlbi-on-map option to the smmuv3 driver.
> When set, the IO_PGTABLE_QUIRK_TLBI_ON_MAP quirk is applied for 
> LPAE tables and the smmuv3 driver sends TLB invalidations on map.
> 
> This mode is useful when running the driver on a guest as it allows
> the virtualizer to trap any change to the translation structures.
> This is similar to the Intel vtd caching mode (CM).
> 
> This is especially needed for VFIO integration integration where
> guest mappings must be applied to the physical IOMMU.
> 
> At the moment the option only is available for DT probing.

I'm really not a fan of this approach. If a virtual IOMMU implementation is
advertising itself as an SMMUv3, then it should adhere to the SMMUv3
architecture and not require non-standard behaviour from the driver. If
we're going to allow that, then we're better off going the extra mile and
using a PV approach. Given that the the SMMU3 architecture does *not*
require TLBI on map, then I don't think we should be quirking our behaviour
in this way. The fact that you only have this implemented for DT is the
canary in the coal mine imo.

Will