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 09:47:47 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Robin Murphy <robin.murphy@....com>,
        Nicolin Chen <nicolinc@...dia.com>,
        "jgg@...dia.com" <jgg@...dia.com>,
        "will@...nel.org" <will@...nel.org>
CC:     "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: Robin Murphy <robin.murphy@....com>
> Sent: Thursday, March 9, 2023 10:49 PM
> > +	case CMDQ_OP_ATC_INV:
> > +		ssid = inv_info->ssid;
> > +		iova = inv_info->range.start;
> > +		size = inv_info->range.last - inv_info->range.start + 1;
> > +		break;
> 
> Can we do any better than multiplying every single ATC_INV command, even
> for random bogus StreamIDs, into multiple commands across every physical
> device? In fact, I'm not entirely confident this isn't problematic, if
> the guest wishes to send invalidations for one device specifically while
> it's put some other device into a state where sending it a command would
> do something bad. At the very least, it's liable to be confusing if the
> guest sends a command for one StreamID but gets an error back for a
> different one.
> 

Or do we need support this cmd at all?

For vt-d we always implicitly invalidate ATC following a iotlb invalidation
request from userspace. Then vIOMMU just treats it as a nop in the
virtual queue.

IMHO a sane iommu driver should always invalidate both iotlb and atc
together. I'm not sure a valid usage where iotlb is invalidated while
atc is left with some stale mappings.