linux-kernel - Re: [Stratos-dev] [PATCH V3 1/3] gpio: Add virtio-gpio driver

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Date:   Mon, 14 Jun 2021 10:56:17 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Arnd Bergmann <arnd@...aro.org>
Cc:     Jean-Philippe Brucker <jean-philippe@...aro.org>,
        Stefan Hajnoczi <stefanha@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Viresh Kumar <vireshk@...nel.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        virtualization@...ts.linux-foundation.org,
        Bartosz Golaszewski <bgolaszewski@...libre.com>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        Stratos Mailing List <stratos-dev@...lists.linaro.org>,
        "Enrico Weigelt, metux IT consult" <info@...ux.net>,
        Jason Wang <jasowang@...hat.com>,
        "Stefano Garzarella --cc virtualization @ lists . linux-foundation . org" 
        <sgarzare@...hat.com>
Subject: Re: [Stratos-dev] [PATCH V3 1/3] gpio: Add virtio-gpio driver

On 11-06-21, 10:34, Arnd Bergmann wrote:
> Extraneous __packed annotations do cause real problems:
> 
> - On architectures without hardware unaligned accesses, the compiler is
>   forced to emit byte load/store instructions, which is slower and breaks
>   atomic updates to shared variables
> 
> - If a function takes a pointer of a packed struct member, and passes that
>   pointer to a function that expects a regular aligned pointer, you
> get undefined
>   behavior. Newer compilers produce a warning if you do that (we currently
>   shut up that warning because there are many false positives in the kernel),
>   but you can also run into CPU exceptions or broken code even on CPUs
>   that do support unaligned accesses when the variable ends up being
>   actually unaligned (as you just told the compiler that it is allowed to do).

I understand that these problems will happen if the structure isn't
aligned, but in this case the structure is aligned properly, just that
we are explicitly telling the compiler to not add any padding (it
won't have added any in here). Is it still harmful ?

-- 
viresh