linux-kernel - RE: [PATCH v6 1/6] docs: IOMMU user API

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Date:   Wed, 29 Jul 2020 01:18:04 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Alex Williamson <alex.williamson@...hat.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>
CC:     "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Joerg Roedel <joro@...tes.org>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        David Woodhouse <dwmw2@...radead.org>,
        "Liu, Yi L" <yi.l.liu@...el.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "Christoph Hellwig" <hch@...radead.org>,
        Jean-Philippe Brucker <jean-philippe@...aro.com>,
        Eric Auger <eric.auger@...hat.com>,
        "Jonathan Corbet" <corbet@....net>
Subject: RE: [PATCH v6 1/6] docs: IOMMU user API

> From: Alex Williamson <alex.williamson@...hat.com>
> Sent: Wednesday, July 29, 2020 3:20 AM
> 
[...]
> > +
> > +For example, IOTLB invalidations should always succeed. There is no
> > +architectural way to report back to the vIOMMU if the UAPI data is
> > +incompatible. If that happens, in order to guarantee IOMMU iosolation,
> 
> isolation
> 
> > +we have to resort to not giving completion status in vIOMMU. This may
> > +result in VM hang.
> > +
> > +For this reason the following IOMMU UAPIs cannot fail without
> > +catastrophic effect:
> > +
> > +1. Free PASID
> > +2. Unbind guest PASID
> > +3. Unbind guest PASID table (SMMU)
> > +4. Cache invalidate
> 
> I'm not entirely sure what we're trying to assert here.  Clearly cache
> invalidation can fail and patch 5/6 goes on to add over a dozen checks
> of the user provided data that return an -errno.  Any user ioctl can
> fail if the user botches the parameters.  So are we just trying to
> explain the feature checking that should allow the user to know
> supported combinations and if they adhere to them, these should not
> fail?  It's not quite worded to that effect.  Thanks,
> 

I guess the above wording is messed by what a UAPI should
behave and whether the vIOMMU reports associated errors.
UAPI can always fail, as you pointed out. vIOMMU may not
have a matching error code though, e.g. on Intel VT-d there is no
error reporting mechanism for cache invalidation. However,
it is not wise to assert UAPI behavior according to vIOMMU
definition. An error is an error. vIOMMU should just react to
UAPI errors according to its architecture definition (e.g. ignore,
forward to guest, hang, etc.). From this matter I feel above
section could better be removed.

Thanks
Kevin