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Tsirkin" , "Arnd Bergmann" Cc: "Jason Wang" , "Xie Yongji" , =?UTF-8?Q?Eugenio_P=C3=A9rez?= , "Xuan Zhuo" , "Marco Crivellari" , "Anders Roxell" , virtualization@lists.linux.dev, linux-kernel@vger.kernel.org Message-Id: <0134edd1-de51-46b3-9ea8-bdbcf48b878f@app.fastmail.com> In-Reply-To: <20260610012421-mutt-send-email-mst@kernel.org> References: <20260213154051.4172275-1-arnd@kernel.org> <20260610012421-mutt-send-email-mst@kernel.org> Subject: Re: [PATCH] [v3] vduse: fix compat handling for VDUSE_IOTLB_GET_FD/VDUSE_VQ_GET_INFO Content-Type: text/plain Content-Transfer-Encoding: 7bit On Wed, Jun 10, 2026, at 07:26, Michael S. Tsirkin wrote: > On Fri, Feb 13, 2026 at 04:40:46PM +0100, Arnd Bergmann wrote: >> + }; >> + __u8 ready; >> + __u8 padding[3]; >> +} __uapi_arch_align; > > what is this __uapi_arch_align supposed to be doing? > > It compiles by luck because gcc thinks it's a global variable. Sorry, that should not have been part of this patch, it is part of the series of changes I was testing when I noticed the problem here, but that other series is unfortunately still not ready for merging. The background here is that I'm adding -Werror=padded to the UAPI checks in usr/include/Makefile, in order to find any instances of data structures with implied padding, and then instead add explicit padding using architecture specific macros. In vduse_vq_info, this includes a final 32 bit of padding on architectures with naturally aligned __u64 but no padding when __u64 has a smaller alignment. The __uapi_arch_align macro in turn is what I add to structures with optional padding, in order to be able to reduce the alignment of a structure when building the kernel with a higher default alignment than userspace (e.g. on m68k with -malign-int). Removing the __uapi_arch_align is the correct fix for the moment. Let me know if you would like me to send a replacement patch without it, a fixup patch on top, or if you will fix it up yourself. Arnd