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Re: [PATCH 04/12] x86: add noreturn in a few more places
- To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Tue, 1 Sep 2026 08:31:15 +0200
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- Cc: Nicola Vetrini <nicola.vetrini@xxxxxxxxxxx>, Teddy Astie <teddy.astie@xxxxxxxxxx>, Roger Pau Monné <roger@xxxxxxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Tue, 01 Sep 2026 06:40:28 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 31.08.2026 21:13, Andrew Cooper wrote:
> On 28/08/2026 8:01 am, Jan Beulich wrote:
>> --- a/xen/arch/x86/traps.c
>> +++ b/xen/arch/x86/traps.c
>> @@ -2304,7 +2304,7 @@ void asmlinkage entry_from_pv(struct cpu
>> case X86_ET_HW_EXC:
>> switch ( vec )
>> {
>> - case X86_EXC_DF: return do_double_fault(regs);
>> + case X86_EXC_DF: do_double_fault(regs); /* noreturn */
>> case X86_EXC_MC: return do_machine_check(regs);
>> }
>> break;
>> @@ -2615,7 +2615,7 @@ void asmlinkage entry_from_xen(struct cp
>> case X86_ET_HW_EXC:
>> switch ( regs->fred_ss.vector )
>> {
>> - case X86_EXC_DF: return do_double_fault(regs);
>> + case X86_EXC_DF: do_double_fault(regs); /* noreturn */
>> case X86_EXC_MC: return do_machine_check(regs);
>> }
>> break;
>
> For starters you're missing a break, and the only reason this isn't a
> compile error is the trailing comment. Second, it's a tailcall anyway.
> There really is nothing unreachable anywhere in this construct.
>
> But by far the most important, it the singular noreturn attribute on
> do_double_fault() (elsewhere, and not visible when reading these two
> functions) which is preventing #DF falling into #MC. This introduces
> fragility which did not exist previously.
>
> do_double_fault() would conditionally return if we ever got around to
> fixing espfix64.
>
> So no - I'm going to insist that Eclair is taught to accept "return
> some_noreturn_fn();" as intentional. It is objectively less fragile
> than the MISRA-preferred option.
Oh, also: For a v2, how much of this change do you demand dropping /
splitting off? Just the two hunks above, or also the one adding noreturn
to do_double_fault()?
Jan
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