linux-kernel - RE: [PATCH v2 3/3] vfio: use __aligned_u64 in struct vfio_device

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Date:   Mon, 11 Sep 2023 06:08:38 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Stefan Hajnoczi <stefanha@...hat.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>
CC:     David Laight <David.Laight@...LAB.COM>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Jason Gunthorpe <jgg@...dia.com>
Subject: RE: [PATCH v2 3/3] vfio: use __aligned_u64 in struct
 vfio_device_ioeventfd

> From: Stefan Hajnoczi <stefanha@...hat.com>
> Sent: Wednesday, August 30, 2023 2:27 AM
> 
> The memory layout of struct vfio_device_ioeventfd is
> architecture-dependent due to a u64 field and a struct size that is not
> a multiple of 8 bytes:
> - On x86_64 the struct size is padded to a multiple of 8 bytes.
> - On x32 the struct size is only a multiple of 4 bytes, not 8.
> - Other architectures may vary.
> 
> Use __aligned_u64 to make memory layout consistent. This reduces the
> chance that 32-bit userspace on a 64-bit kernel breakage.
> 
> This patch increases the struct size on x32 but this is safe because of
> the struct's argsz field. The kernel may grow the struct as long as it
> still supports smaller argsz values from userspace (e.g. applications
> compiled against older kernel headers).
> 
> The code that uses struct vfio_device_ioeventfd already works correctly
> when the struct size grows, so only the struct definition needs to be
> changed.
> 
> Suggested-by: Jason Gunthorpe <jgg@...dia.com>
> Reviewed-by: Jason Gunthorpe <jgg@...dia.com>
> Signed-off-by: Stefan Hajnoczi <stefanha@...hat.com>

Reviewed-by: Kevin Tian <kevin.tian@...el.com>