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

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Date:   Tue, 21 Mar 2023 08:48:31 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>
Cc:     Nicolin Chen <nicolinc@...dia.com>,
        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

On Tue, Mar 21, 2023 at 08:34:00AM +0000, Tian, Kevin wrote:

> > > Rephrasing that to put into a design: the IOCTL would pass a
> > > user pointer to the queue, the size of the queue, then a head
> > > pointer and a tail pointer? Then the kernel reads out all the
> > > commands between the head and the tail and handles all those
> > > invalidation commands only?
> > 
> > Yes, that is one possible design
> 
> If we cannot have the short path in the kernel then I'm not sure the
> value of using native format and queue in the uAPI. Batching can
> be enabled over any format.

SMMUv3 will have a hardware short path where the HW itself runs the
VM's command queue and does this logic.

So I like the symmetry of the SW path being close to that.

> Btw probably a dumb question. The current invalidation IOCTL is
> per hwpt. If picking a native format does it suggest making the IOCTL
> per iommufd given native format is per IOMMU and could carry
> scope bigger than a hwpt.

At least on SMMUv3 it depends on what happens with VMID.

If we can tie the VMID to the iommu_domain then the invalidation has
to flow through the iommu_domain to pick up the VMID.

If the VMID is tied to the entire iommufd_ctx then it can flow
independently.

Jason