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Date:   Wed, 4 Jan 2023 08:16:20 -0400
From:   Jason Gunthorpe <jgg@ziepe.ca>
To:     Niklas Schnelle <schnelle@linux.ibm.com>
Cc:     Alex Williamson <alex.williamson@redhat.com>,
        Cornelia Huck <cohuck@redhat.com>, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-s390@vger.kernel.org,
        Matthew Rosato <mjrosato@linux.ibm.com>,
        Pierre Morel <pmorel@linux.ibm.com>,
        Christian =?utf-8?Q?Borntr=C3=A4ger?= <borntraeger@linux.ibm.com>
Subject: Re: [PATCH 1/1] vfio/type1: Respect IOMMU reserved regions in
 vfio_test_domain_fgsp()
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On Wed, Jan 04, 2023 at 10:52:55AM +0100, Niklas Schnelle wrote:

> The problem manifests only with ISM devices which are a special s390
> virtual PCI device that is implemented in the machine hypervisor. This
> device is used for high speed cross-LPAR (Logical Partition)
> communication, basically it allows two LPARs that previously exchanged
> an authentication token to memcpy between their partitioned memory
> using the virtual device. For copying a receiving LPAR will IOMMU map a
> region of memory for the ISM device that it will allow DMAing into
> (memcpy by the hypervisor). All other regions remain unmapped and thus
> inaccessible. In preparation the device  emulation in the machine
> hypervisor intercepts the IOTLB flush and looks at the IOMMU
> translation tables performing e.g. size and alignment checks I presume,
> one of these checks against the start/end DMA boundaries. This check
> fails which leads to the virtual ISM device being put into an error
> state. Being in an error state it then fails to be initialized by the
> guest driver later on.

You could rephrase this as saying that the S390 map operation doesn't
check for bounds so mapping in a reserved region doesn't fail, but
errors the HW.

Which seems reasonable to me

Jason