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

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Date:   Fri, 17 Mar 2023 09:24:16 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Jason Gunthorpe <jgg@...dia.com>,
        Robin Murphy <robin.murphy@....com>
CC:     Nicolin Chen <nicolinc@...dia.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: Thursday, March 9, 2023 11:31 PM
> 
> > Also, perhaps I've overlooked something obvious, but what's the
> procedure
> > for reflecting illegal commands back to userspace? Some of the things we're
> > silently ignoring here would be expected to raise CERROR_ILL. Same goes
> for
> > all the other fault events which may occur due to invalid S1 config, come to
> > think of it.
> 
> Perhaps the ioctl should fail and the userpace viommu should inject
> this CERROR_ILL?
> 
> But I'm also wondering if we are making a mistake to not just have the
> kernel driver to expose a SW work queue in its native format and the
> ioctl is only just 'read the queue'. Then it could (asynchronously!)
> push back answers, real or emulated, as well, including all error
> indications.
> 
> I think we got down this synchronous one-ioctl-per-invalidation path
> because that was what the original generic stuff wanted to do. Is it
> what we really want? Kevin what is your perspective?
> 

That's an interesting idea. I think the original synchronous model
also matches how intel-iommu driver works today. In most time
it does synchronous one-invalidation at one time. 

Another problem is how to map invalidation scope in native descriptor
format to affected devices.

VT-d allows per-DID invalidation. This needs extra information to map
vDID to affected devices in the kernel.

It also allows a global invalidation type which invalidate all vDIDs. This
might be easy by simply looping every device bound to the iommufd_ctx.