linux-kernel - Re: [PATCH v3 0/3] vfio: use __aligned

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Date:   Thu, 28 Sep 2023 15:12:52 -0600
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Stefan Hajnoczi <stefanha@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Jason Gunthorpe <jgg@...pe.ca>,
        David Laight <David.Laight@...LAB.COM>,
        "Tian, Kevin" <kevin.tian@...el.com>
Subject: Re: [PATCH v3 0/3] vfio: use __aligned_u64 for ioctl structs

On Mon, 18 Sep 2023 16:56:14 -0400
Stefan Hajnoczi <stefanha@...hat.com> wrote:

> v3:
> - Remove the output struct sizing code that copied out zeroed fields at the end
>   of the struct. Alex pointed out that new fields (or repurposing a field that
>   was previously reserved) must be guarded by a flag and this means userspace
>   won't access those fields when they are absent.
> 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]
> - 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@...dia.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.
> 
> Stefan Hajnoczi (3):
>   vfio: trivially use __aligned_u64 for ioctl structs
>   vfio: use __aligned_u64 in struct vfio_device_gfx_plane_info
>   vfio: use __aligned_u64 in struct vfio_device_ioeventfd
> 
>  include/uapi/linux/vfio.h        | 26 ++++++++++++++------------
>  drivers/gpu/drm/i915/gvt/kvmgt.c |  2 +-
>  samples/vfio-mdev/mbochs.c       |  2 +-
>  samples/vfio-mdev/mdpy.c         |  2 +-
>  4 files changed, 17 insertions(+), 15 deletions(-)
> 

Applied to vfio next branch for v6.7.  Thanks,

Alex