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Re: [PATCH] do_multicall and MISRA Rule 8.3
- To: Stefano Stabellini <sstabellini@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Thu, 23 Nov 2023 09:30:36 +0100
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- Cc: federico.serafini@xxxxxxxxxxx, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, Julien Grall <julien@xxxxxxx>, Bertrand Marquis <bertrand.marquis@xxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Thu, 23 Nov 2023 08:30:43 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 22.11.2023 22:46, Stefano Stabellini wrote:
> Two out of three do_multicall definitions/declarations use uint32_t as
> type for the "nr_calls" parameters. Change the third one to be
> consistent with the other two.
>
> Link:
> https://lore.kernel.org/xen-devel/7e3abd4c0ef5127a07a60de1bf090a8aefac8e5c.1692717906.git.federico.serafini@xxxxxxxxxxx/
> Link:
> https://lore.kernel.org/xen-devel/alpine.DEB.2.22.394.2308251502430.6458@ubuntu-linux-20-04-desktop/
> Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxx>
> ---
> Note that a previous discussion showed disagreement between maintainers
> on this topic. The source of disagreements are that we don't want to
> change a guest-visible ABI and we haven't properly documented how to use
> types for guest ABIs.
>
> As an example, fixed-width types have the advantage of being explicit
> about their size but sometimes register-size types are required (e.g.
> unsigned long). The C specification says little about the size of
> unsigned long and today, and we even use unsigned int in guest ABIs
> without specifying the expected width of unsigned int on the various
> arches. As Jan pointed out, in Xen we assume sizeof(int) >= 4, but
> that's not written anywhere as far as I can tell.
>
> I think the appropriate solution would be to document properly our
> expectations of both fixed-width and non-fixed-width types, and how to
> use them for guest-visible ABIs.
>
> In this patch I used uint32_t for a couple of reasons:
> - until we have better documentation, I feel more confident in using
> explicitly-sized integers in guest-visible ABIs
I disagree with this way of looking at it. Guests don't invoke these
functions directly, and our assembly code sitting in between already is
expected to (and does) guarantee that (in the case here) unsigned int
would be okay to use (as would be unsigned long, but at least on x86
that's slightly less efficient), in line with what ./CODING_STYLE says.
Otoh structure definitions in the public interface of course need to
use fixed with types (and still doesn't properly do so in a few cases).
Jan
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