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    SubjectRE: [PATCH v2 0/3] vfio: use __aligned_u64 for ioctl structs
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    > -----Original Message-----
    > From: David Laight <David.Laight@ACULAB.COM>
    > Sent: 29 August 2023 22:10
    > To: 'Stefan Hajnoczi' <stefanha@redhat.com>; kvm@vger.kernel.org
    > Cc: linux-kernel@vger.kernel.org; Tian, Kevin <kevin.tian@intel.com>; Alex Williamson
    > <alex.williamson@redhat.com>; Jason Gunthorpe <jgg@ziepe.ca>
    > Subject: RE: [PATCH v2 0/3] vfio: use __aligned_u64 for ioctl structs
    >
    > From: Stefan Hajnoczi
    > > Sent: 29 August 2023 19:27
    > >
    > > v2:
    > > - Rebased onto https://github.com/awilliam/linux-vfio.git next to get the
    > > vfio_iommu_type1_info pad field [Kevin]
    > > - Fixed min(minsz, sizeof(dmabuf)) -> min(dmabuf.argsz, sizeof(dmabuf)) [Jason, Kevin]
    >
    > You managed to use min_t() instead of fixing the types to match.
    >
    > > - Squashed Patch 3 (vfio_iommu_type1_info) into Patch 1 since it is trivial now
    > > that the padding field is already there.
    > >
    > > Jason Gunthorpe <jgg@nvidia.com> pointed out that u64 VFIO ioctl struct fields
    > > have architecture-dependent alignment. iommufd already uses __aligned_u64 to
    > > avoid this problem.
    > >
    > > See the __aligned_u64 typedef in <uapi/linux/types.h> for details on why it is
    > > a good idea for kernel<->user interfaces.
    > >
    > > This series modifies the VFIO ioctl structs to use __aligned_u64. Some of the
    > > changes preserve the existing memory layout on all architectures, so I put them
    > > together into the first patch. The remaining patches are for structs where
    > > explanation is necessary about why changing the memory layout does not break
    > > the uapi.
    >
    > But you are extending a field in the middle of the uapi structure.
    > This completely breaks any applications.

    I've had a closer look this morning.
    Your explanations aren't very good.
    The structures all have the 64bit fields on their natural boundary
    so the memory layout isn't really changed - just extra padding
    at the end.

    David

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