linux-kernel - RE: [PATCH v2 17/19] iommu/arm-smmu-v3: Add arm_smmu_viommu_cache

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Message-ID: <BN9PR11MB5276D3685466B590F36318AA8C652@BN9PR11MB5276.namprd11.prod.outlook.com>
Date: Fri, 13 Sep 2024 02:33:59 +0000
From: "Tian, Kevin" <kevin.tian@...el.com>
To: Jason Gunthorpe <jgg@...dia.com>
CC: Nicolin Chen <nicolinc@...dia.com>, "will@...nel.org" <will@...nel.org>,
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	<smostafa@...gle.com>, "Liu, Yi L" <yi.l.liu@...el.com>
Subject: RE: [PATCH v2 17/19] iommu/arm-smmu-v3: Add
 arm_smmu_viommu_cache_invalidate

> From: Jason Gunthorpe <jgg@...dia.com>
> Sent: Thursday, September 12, 2024 7:08 AM
> 
> On Wed, Sep 11, 2024 at 08:13:01AM +0000, Tian, Kevin wrote:
> 
> > Probably there is a good reason e.g. for simplification or better
> > aligned with hw accel stuff. But it's not explained clearly so far.
> 
> Probably the most concrete thing is if you have a direct assignment
> invalidation queue (ie DMA'd directly by HW) then it only applies to a
> single pIOMMU and invalidation commands placed there are unavoidably
> limited in scope.
> 
> This creates a representation problem, if we have a vIOMMU that spans
> many pIOMMUs but invalidations do some subset how to do we model
> that. Just saying the vIOMMU is linked to the pIOMMU solves this
> nicely.
> 

yes that is a good reason.

btw do we expect the VMM to try-and-fail when deciding whether a
new vIOMMU object is required when creating a new vdev?