linux-kernel - RE: [PATCH v1 14/14] iommu/arm-smmu-v3: Add arm_smmu_cache_invalidate

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Message-ID: <BN9PR11MB52768F4D3E21C5231C1A04D38CBD9@BN9PR11MB5276.namprd11.prod.outlook.com>
Date:   Fri, 17 Mar 2023 09:41:34 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Jason Gunthorpe <jgg@...dia.com>,
        Nicolin Chen <nicolinc@...dia.com>
CC:     Robin Murphy <robin.murphy@....com>,
        "will@...nel.org" <will@...nel.org>,
        "eric.auger@...hat.com" <eric.auger@...hat.com>,
        "baolu.lu@...ux.intel.com" <baolu.lu@...ux.intel.com>,
        "joro@...tes.org" <joro@...tes.org>,
        "shameerali.kolothum.thodi@...wei.com" 
        <shameerali.kolothum.thodi@...wei.com>,
        "jean-philippe@...aro.org" <jean-philippe@...aro.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v1 14/14] iommu/arm-smmu-v3: Add
 arm_smmu_cache_invalidate_user

> From: Jason Gunthorpe <jgg@...dia.com>
> Sent: Saturday, March 11, 2023 12:20 AM
> 
> What I'm broadly thinking is if we have to make the infrastructure for
> VCMDQ HW accelerated invalidation then it is not a big step to also
> have the kernel SW path use the same infrastructure just with a CPU
> wake up instead of a MMIO poke.
> 
> Ie we have a SW version of VCMDQ to speed up SMMUv3 cases without HW
> support.
> 

I thought about this in VT-d context. Looks there are some difficulties.

The most prominent one is that head/tail of the VT-d invalidation queue
are in MMIO registers. Handling it in kernel iommu driver suggests
reading virtual tail register and updating virtual head register. Kind of 
moving some vIOMMU awareness into the kernel which, iirc, is not
a welcomed model.

vhost doesn't have this problem as its vring structure fully resides in
memory including ring tail/head. As long as kernel vhost driver understands
the structure and can send/receive notification to/from kvm then the
in-kernel acceleration works seamlessly.

Not sure whether SMMU has similar obstacle as VT-d. But this is my
impression why vhost-iommu is preferred when talking about such
optimization before.