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Re: [PATCH v2] x86/altcall: use an union as register type for function parameters
- To: Roger Pau Monne <roger.pau@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Thu, 22 Feb 2024 17:55:00 +0100
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- Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Matthew Grooms <mgrooms@xxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Thu, 22 Feb 2024 16:55:10 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 22.02.2024 17:44, Roger Pau Monne wrote:
> --- a/xen/arch/x86/include/asm/alternative.h
> +++ b/xen/arch/x86/include/asm/alternative.h
> @@ -167,9 +167,25 @@ extern void alternative_branches(void);
> #define ALT_CALL_arg5 "r8"
> #define ALT_CALL_arg6 "r9"
>
> +#ifdef CONFIG_CC_IS_CLANG
> +/*
> + * Use an union with an unsigned long in order to prevent clang from
> skipping a
> + * possible truncation of the value. By using the union any truncation is
> + * carried before the call instruction.
> + * https://github.com/llvm/llvm-project/issues/82598
> + */
I think it needs saying that this is relying on compiler behavior not
mandated by the standard, thus explaining why it's restricted to
Clang (down the road we may even want to restrict to old versions,
assuming they fix the issue at some point). Plus also giving future
readers a clear understanding that if something breaks with this, it's
not really a surprise.
Aiui this bug is only a special case of the other, much older one, so
referencing that one here too would seem advisable.
As a nit: I think it is "a union" (spelling according to pronunciation),
and I guess the title could now do with saying "optionally" or
mentioning Clang or some such.
Jan
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