linux-kernel - Re: [PATCH v3 1/1] vfio/type1: Respect IOMMU reserved regions in vfio_test_domain

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Message-ID: <b652c69f-96e4-3584-7559-33cb8a8ca4f0@linux.ibm.com>
Date:   Tue, 10 Jan 2023 12:06:47 -0500
From:   Matthew Rosato <mjrosato@...ux.ibm.com>
To:     Niklas Schnelle <schnelle@...ux.ibm.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        Cornelia Huck <cohuck@...hat.com>
Cc:     Jason Gunthorpe <jgg@...pe.ca>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org,
        Pierre Morel <pmorel@...ux.ibm.com>,
        Christian Bornträger <borntraeger@...ux.ibm.com>
Subject: Re: [PATCH v3 1/1] vfio/type1: Respect IOMMU reserved regions in
 vfio_test_domain_fgsp()

On 1/10/23 11:44 AM, Niklas Schnelle wrote:
> Since commit cbf7827bc5dc ("iommu/s390: Fix potential s390_domain
> aperture shrinking") the s390 IOMMU driver uses reserved regions for the
> system provided DMA ranges of PCI devices. Previously it reduced the
> size of the IOMMU aperture and checked it on each mapping operation.
> On current machines the system denies use of DMA addresses below 2^32 for
> all PCI devices.
> 
> Usually mapping IOVAs in a reserved regions is harmless until a DMA
> actually tries to utilize the mapping. However on s390 there is
> a virtual PCI device called ISM which is implemented in firmware and
> used for cross LPAR communication. Unlike real PCI devices this device
> does not use the hardware IOMMU but inspects IOMMU translation tables
> directly on IOTLB flush (s390 RPCIT instruction). If it detects IOVA
> mappings outside the allowed ranges it goes into an error state. This
> error state then causes the device to be unavailable to the KVM guest.
> 
> Analysing this we found that vfio_test_domain_fgsp() maps 2 pages at DMA
> address 0 irrespective of the IOMMUs reserved regions. Even if usually
> harmless this seems wrong in the general case so instead go through the
> freshly updated IOVA list and try to find a range that isn't reserved,
> and fits 2 pages, is PAGE_SIZE * 2 aligned. If found use that for
> testing for fine grained super pages.
> 
> Fixes: af029169b8fd ("vfio/type1: Check reserved region conflict and update iova list")
> Signed-off-by: Niklas Schnelle <schnelle@...ux.ibm.com>

Looks good to me still (also re-tested ISM passthrough on s390).  Thanks!

Reviewed-by: Matthew Rosato <mjrosato@...ux.ibm.com>