[PATCH] xen: address violation of MISRA C Rule 11.1

[PATCH] xen: address violation of MISRA C Rule 11.1

[Alessandro Zucchelli]

Rule 11.1 states as following: "Conversions shall not be performed
between a pointer to a function and any other type".

Functions "__machine_restart" and "__machine_halt" in "x86/shutdown.c"
and "halt_this_cpu" in "arm/shutdown.c" are defined as noreturn
functions and subsequently passed as parameters to function calls.
This violates the rule in Clang, where the "noreturn" attribute is
considered part of the function"s type. By removing the "noreturn"
attribbute and replacing it with uses of the ASSERT_UNREACHABLE macro,
these violations are addressed.

Signed-off-by: Alessandro Zucchelli <alessandro.zucchelli@bugseng.com>
---
 xen/arch/arm/shutdown.c | 3 ++-
 xen/arch/x86/shutdown.c | 7 +++++--
 2 files changed, 7 insertions(+), 3 deletions(-)

diff --git a/xen/arch/arm/shutdown.c b/xen/arch/arm/shutdown.c
index c9778e5786..e679ae8d72 100644
--- a/xen/arch/arm/shutdown.c
+++ b/xen/arch/arm/shutdown.c
@@ -8,7 +8,7 @@
 #include <asm/platform.h>
 #include <asm/psci.h>
 
-static void noreturn halt_this_cpu(void *arg)
+static void halt_this_cpu(void *arg)
 {
     local_irq_disable();
     /* Make sure the write happens before we sleep forever */
@@ -38,6 +38,7 @@ void machine_halt(void)
     /* Alternative halt procedure */
     platform_poweroff();
     halt_this_cpu(NULL);
+    ASSERT_UNREACHABLE();
 }
 
 void machine_restart(unsigned int delay_millisecs)
diff --git a/xen/arch/x86/shutdown.c b/xen/arch/x86/shutdown.c
index 902076cf67..b684e19754 100644
--- a/xen/arch/x86/shutdown.c
+++ b/xen/arch/x86/shutdown.c
@@ -118,7 +118,7 @@ static inline void kb_wait(void)
             break;
 }
 
-static void noreturn cf_check __machine_halt(void *unused)
+static void cf_check __machine_halt(void *unused)
 {
     local_irq_disable();
 
@@ -127,6 +127,7 @@ static void noreturn cf_check __machine_halt(void *unused)
 
     for ( ; ; )
         halt();
+    ASSERT_UNREACHABLE();
 }
 
 void machine_halt(void)
@@ -141,6 +142,7 @@ void machine_halt(void)
     }
 
     __machine_halt(NULL);
+    ASSERT_UNREACHABLE();
 }
 
 static void default_reboot_type(void)
@@ -520,9 +522,10 @@ static int __init cf_check reboot_init(void)
 }
 __initcall(reboot_init);
 
-static void cf_check noreturn __machine_restart(void *pdelay)
+static void cf_check __machine_restart(void *pdelay)
 {
     machine_restart(*(unsigned int *)pdelay);
+    ASSERT_UNREACHABLE();
 }
 
 void machine_restart(unsigned int delay_millisecs)
-- 
2.43.0

Re: [PATCH] xen: address violation of MISRA C Rule 11.1

[Jan Beulich]

On 11.12.2024 12:02, Alessandro Zucchelli wrote:
> Rule 11.1 states as following: "Conversions shall not be performed
> between a pointer to a function and any other type".
> 
> Functions "__machine_restart" and "__machine_halt" in "x86/shutdown.c"
> and "halt_this_cpu" in "arm/shutdown.c" are defined as noreturn
> functions and subsequently passed as parameters to function calls.
> This violates the rule in Clang, where the "noreturn" attribute is
> considered part of the function"s type.

I'm unaware of build issues with Clang, hence can you clarify how Clang's
view comes into play here? In principle various attributes ought to be
part of a function's type; iirc that's also the case for gcc. Yet how
that matters to Eclair is still entirely unclear to me.

> By removing the "noreturn"
> attribbute and replacing it with uses of the ASSERT_UNREACHABLE macro,
> these violations are addressed.

Papered over, I'd say. What about release builds, for example?

Deleting the attribute also has a clear downside documentation-wise. If
we really mean to remove them from what the compiler gets to see, I think
we ought to still retain them in commented-out shape.

Jan

Re: [PATCH] xen: address violation of MISRA C Rule 11.1

[Stefano Stabellini]

On Wed, 11 Dec 2024, Jan Beulich wrote:
> On 11.12.2024 12:02, Alessandro Zucchelli wrote:
> > Rule 11.1 states as following: "Conversions shall not be performed
> > between a pointer to a function and any other type".
> > 
> > Functions "__machine_restart" and "__machine_halt" in "x86/shutdown.c"
> > and "halt_this_cpu" in "arm/shutdown.c" are defined as noreturn
> > functions and subsequently passed as parameters to function calls.
> > This violates the rule in Clang, where the "noreturn" attribute is
> > considered part of the function"s type.
> 
> I'm unaware of build issues with Clang, hence can you clarify how Clang's
> view comes into play here? In principle various attributes ought to be
> part of a function's type; iirc that's also the case for gcc. Yet how
> that matters to Eclair is still entirely unclear to me.
> 
> > By removing the "noreturn"
> > attribbute and replacing it with uses of the ASSERT_UNREACHABLE macro,
> > these violations are addressed.
> 
> Papered over, I'd say. What about release builds, for example?
> 
> Deleting the attribute also has a clear downside documentation-wise. If
> we really mean to remove them from what the compiler gets to see, I think
> we ought to still retain them in commented-out shape.

Another option would be to #define noreturn to nothing for ECLAIR builds ?

Re: [PATCH] xen: address violation of MISRA C Rule 11.1

[Jan Beulich]

On 12.12.2024 03:27, Stefano Stabellini wrote:
> On Wed, 11 Dec 2024, Jan Beulich wrote:
>> On 11.12.2024 12:02, Alessandro Zucchelli wrote:
>>> Rule 11.1 states as following: "Conversions shall not be performed
>>> between a pointer to a function and any other type".
>>>
>>> Functions "__machine_restart" and "__machine_halt" in "x86/shutdown.c"
>>> and "halt_this_cpu" in "arm/shutdown.c" are defined as noreturn
>>> functions and subsequently passed as parameters to function calls.
>>> This violates the rule in Clang, where the "noreturn" attribute is
>>> considered part of the function"s type.
>>
>> I'm unaware of build issues with Clang, hence can you clarify how Clang's
>> view comes into play here? In principle various attributes ought to be
>> part of a function's type; iirc that's also the case for gcc. Yet how
>> that matters to Eclair is still entirely unclear to me.
>>
>>> By removing the "noreturn"
>>> attribbute and replacing it with uses of the ASSERT_UNREACHABLE macro,
>>> these violations are addressed.
>>
>> Papered over, I'd say. What about release builds, for example?
>>
>> Deleting the attribute also has a clear downside documentation-wise. If
>> we really mean to remove them from what the compiler gets to see, I think
>> we ought to still retain them in commented-out shape.
> 
> Another option would be to #define noreturn to nothing for ECLAIR builds ?

That again would feel like papering over things. Plus I don't know if that's
an option at all.

Jan

Re: [PATCH] xen: address violation of MISRA C Rule 11.1

[Stefano Stabellini]

On Thu, 12 Dec 2024, Jan Beulich wrote:
> On 12.12.2024 03:27, Stefano Stabellini wrote:
> > On Wed, 11 Dec 2024, Jan Beulich wrote:
> >> On 11.12.2024 12:02, Alessandro Zucchelli wrote:
> >>> Rule 11.1 states as following: "Conversions shall not be performed
> >>> between a pointer to a function and any other type".
> >>>
> >>> Functions "__machine_restart" and "__machine_halt" in "x86/shutdown.c"
> >>> and "halt_this_cpu" in "arm/shutdown.c" are defined as noreturn
> >>> functions and subsequently passed as parameters to function calls.
> >>> This violates the rule in Clang, where the "noreturn" attribute is
> >>> considered part of the function"s type.
> >>
> >> I'm unaware of build issues with Clang, hence can you clarify how Clang's
> >> view comes into play here? In principle various attributes ought to be
> >> part of a function's type; iirc that's also the case for gcc. Yet how
> >> that matters to Eclair is still entirely unclear to me.
> >>
> >>> By removing the "noreturn"
> >>> attribbute and replacing it with uses of the ASSERT_UNREACHABLE macro,
> >>> these violations are addressed.
> >>
> >> Papered over, I'd say. What about release builds, for example?
> >>
> >> Deleting the attribute also has a clear downside documentation-wise. If
> >> we really mean to remove them from what the compiler gets to see, I think
> >> we ought to still retain them in commented-out shape.
> > 
> > Another option would be to #define noreturn to nothing for ECLAIR builds ?
> 
> That again would feel like papering over things. Plus I don't know if that's
> an option at all.

What is "papering over" and what is a "nice solution" is often up to the
personal opinions.

From my point of view, Alessandro's patch doesn't make the code worse.
The ASSERT_UNREACHABLE solution is OK. I do agree with you that it
should not be required for us to remove "noreturn", but I don't think we
have used it consistently anyway across the Xen codebase.
ASSERT_UNREACHABLE is also a form of documentation that the function
does not return.

In conclusion, I think all three options are acceptable:
1) this patch as is
2) this patch plus /* noreturn */ as a comment
3) #define noreturn to nothing just for ECLAIR builds

I don't mind either way, maybe option 2) is the best compromise.

Re: [PATCH] xen: address violation of MISRA C Rule 11.1

[Jan Beulich]

On 13.12.2024 01:53, Stefano Stabellini wrote:
> On Thu, 12 Dec 2024, Jan Beulich wrote:
>> On 12.12.2024 03:27, Stefano Stabellini wrote:
>>> On Wed, 11 Dec 2024, Jan Beulich wrote:
>>>> On 11.12.2024 12:02, Alessandro Zucchelli wrote:
>>>>> Rule 11.1 states as following: "Conversions shall not be performed
>>>>> between a pointer to a function and any other type".
>>>>>
>>>>> Functions "__machine_restart" and "__machine_halt" in "x86/shutdown.c"
>>>>> and "halt_this_cpu" in "arm/shutdown.c" are defined as noreturn
>>>>> functions and subsequently passed as parameters to function calls.
>>>>> This violates the rule in Clang, where the "noreturn" attribute is
>>>>> considered part of the function"s type.
>>>>
>>>> I'm unaware of build issues with Clang, hence can you clarify how Clang's
>>>> view comes into play here? In principle various attributes ought to be
>>>> part of a function's type; iirc that's also the case for gcc. Yet how
>>>> that matters to Eclair is still entirely unclear to me.
>>>>
>>>>> By removing the "noreturn"
>>>>> attribbute and replacing it with uses of the ASSERT_UNREACHABLE macro,
>>>>> these violations are addressed.
>>>>
>>>> Papered over, I'd say. What about release builds, for example?
>>>>
>>>> Deleting the attribute also has a clear downside documentation-wise. If
>>>> we really mean to remove them from what the compiler gets to see, I think
>>>> we ought to still retain them in commented-out shape.
>>>
>>> Another option would be to #define noreturn to nothing for ECLAIR builds ?
>>
>> That again would feel like papering over things. Plus I don't know if that's
>> an option at all.
> 
> What is "papering over" and what is a "nice solution" is often up to the
> personal opinions.
> 
> From my point of view, Alessandro's patch doesn't make the code worse.
> The ASSERT_UNREACHABLE solution is OK. I do agree with you that it
> should not be required for us to remove "noreturn", but I don't think we
> have used it consistently anyway across the Xen codebase.
> ASSERT_UNREACHABLE is also a form of documentation that the function
> does not return.
> 
> In conclusion, I think all three options are acceptable:
> 1) this patch as is
> 2) this patch plus /* noreturn */ as a comment
> 3) #define noreturn to nothing just for ECLAIR builds
> 
> I don't mind either way, maybe option 2) is the best compromise.

The variant with least impact on what we currently have (generated code
wise) is 3), though, which hence would be my preference (well, not exactly
a preference, but the least bad one).

Jan

Re: [PATCH] xen: address violation of MISRA C Rule 11.1

[Alessandro Zucchelli]

On 2024-12-13 11:08, Jan Beulich wrote:
> On 13.12.2024 01:53, Stefano Stabellini wrote:
>> On Thu, 12 Dec 2024, Jan Beulich wrote:
>>> On 12.12.2024 03:27, Stefano Stabellini wrote:
>>>> On Wed, 11 Dec 2024, Jan Beulich wrote:
>>>>> On 11.12.2024 12:02, Alessandro Zucchelli wrote:
>>>>>> Rule 11.1 states as following: "Conversions shall not be performed
>>>>>> between a pointer to a function and any other type".
>>>>>> 
>>>>>> Functions "__machine_restart" and "__machine_halt" in 
>>>>>> "x86/shutdown.c"
>>>>>> and "halt_this_cpu" in "arm/shutdown.c" are defined as noreturn
>>>>>> functions and subsequently passed as parameters to function calls.
>>>>>> This violates the rule in Clang, where the "noreturn" attribute is
>>>>>> considered part of the function"s type.
>>>>> 
>>>>> I'm unaware of build issues with Clang, hence can you clarify how 
>>>>> Clang's
>>>>> view comes into play here? In principle various attributes ought to 
>>>>> be
>>>>> part of a function's type; iirc that's also the case for gcc. Yet 
>>>>> how
>>>>> that matters to Eclair is still entirely unclear to me.
>>>>> 
>>>>>> By removing the "noreturn"
>>>>>> attribbute and replacing it with uses of the ASSERT_UNREACHABLE 
>>>>>> macro,
>>>>>> these violations are addressed.
>>>>> 
>>>>> Papered over, I'd say. What about release builds, for example?
>>>>> 
>>>>> Deleting the attribute also has a clear downside 
>>>>> documentation-wise. If
>>>>> we really mean to remove them from what the compiler gets to see, I 
>>>>> think
>>>>> we ought to still retain them in commented-out shape.
>>>> 
>>>> Another option would be to #define noreturn to nothing for ECLAIR 
>>>> builds ?
>>> 
>>> That again would feel like papering over things. Plus I don't know if 
>>> that's
>>> an option at all.
>> 
>> What is "papering over" and what is a "nice solution" is often up to 
>> the
>> personal opinions.
>> 
>> From my point of view, Alessandro's patch doesn't make the code worse.
>> The ASSERT_UNREACHABLE solution is OK. I do agree with you that it
>> should not be required for us to remove "noreturn", but I don't think 
>> we
>> have used it consistently anyway across the Xen codebase.
>> ASSERT_UNREACHABLE is also a form of documentation that the function
>> does not return.
>> 
>> In conclusion, I think all three options are acceptable:
>> 1) this patch as is
>> 2) this patch plus /* noreturn */ as a comment
>> 3) #define noreturn to nothing just for ECLAIR builds
>> 
>> I don't mind either way, maybe option 2) is the best compromise.
> 
> The variant with least impact on what we currently have (generated code
> wise) is 3), though, which hence would be my preference (well, not 
> exactly
> a preference, but the least bad one).

Another option could be to encapsulate these function pointer casts as 
follows:
#define REMOVE_NORETURN(x) (void(*)(void*))(x)
This approach allows us to retain the noreturn attribute and the 
associated optimizations;
note that the encapsulating macro will need to be deviated then.

-- 
Alessandro Zucchelli, B.Sc.

Software Engineer, BUGSENG (https://bugseng.com)

Re: [PATCH] xen: address violation of MISRA C Rule 11.1

[Stefano Stabellini]

On Fri, 13 Dec 2024, Alessandro Zucchelli wrote:
> On 2024-12-13 11:08, Jan Beulich wrote:
> > On 13.12.2024 01:53, Stefano Stabellini wrote:
> > > On Thu, 12 Dec 2024, Jan Beulich wrote:
> > > > On 12.12.2024 03:27, Stefano Stabellini wrote:
> > > > > On Wed, 11 Dec 2024, Jan Beulich wrote:
> > > > > > On 11.12.2024 12:02, Alessandro Zucchelli wrote:
> > > > > > > Rule 11.1 states as following: "Conversions shall not be performed
> > > > > > > between a pointer to a function and any other type".
> > > > > > > 
> > > > > > > Functions "__machine_restart" and "__machine_halt" in
> > > > > > > "x86/shutdown.c"
> > > > > > > and "halt_this_cpu" in "arm/shutdown.c" are defined as noreturn
> > > > > > > functions and subsequently passed as parameters to function calls.
> > > > > > > This violates the rule in Clang, where the "noreturn" attribute is
> > > > > > > considered part of the function"s type.
> > > > > > 
> > > > > > I'm unaware of build issues with Clang, hence can you clarify how
> > > > > > Clang's
> > > > > > view comes into play here? In principle various attributes ought to
> > > > > > be
> > > > > > part of a function's type; iirc that's also the case for gcc. Yet
> > > > > > how
> > > > > > that matters to Eclair is still entirely unclear to me.
> > > > > > 
> > > > > > > By removing the "noreturn"
> > > > > > > attribbute and replacing it with uses of the ASSERT_UNREACHABLE
> > > > > > > macro,
> > > > > > > these violations are addressed.
> > > > > > 
> > > > > > Papered over, I'd say. What about release builds, for example?
> > > > > > 
> > > > > > Deleting the attribute also has a clear downside documentation-wise.
> > > > > > If
> > > > > > we really mean to remove them from what the compiler gets to see, I
> > > > > > think
> > > > > > we ought to still retain them in commented-out shape.
> > > > > 
> > > > > Another option would be to #define noreturn to nothing for ECLAIR
> > > > > builds ?
> > > > 
> > > > That again would feel like papering over things. Plus I don't know if
> > > > that's
> > > > an option at all.
> > > 
> > > What is "papering over" and what is a "nice solution" is often up to the
> > > personal opinions.
> > > 
> > > From my point of view, Alessandro's patch doesn't make the code worse.
> > > The ASSERT_UNREACHABLE solution is OK. I do agree with you that it
> > > should not be required for us to remove "noreturn", but I don't think we
> > > have used it consistently anyway across the Xen codebase.
> > > ASSERT_UNREACHABLE is also a form of documentation that the function
> > > does not return.
> > > 
> > > In conclusion, I think all three options are acceptable:
> > > 1) this patch as is
> > > 2) this patch plus /* noreturn */ as a comment
> > > 3) #define noreturn to nothing just for ECLAIR builds
> > > 
> > > I don't mind either way, maybe option 2) is the best compromise.
> > 
> > The variant with least impact on what we currently have (generated code
> > wise) is 3), though, which hence would be my preference (well, not exactly
> > a preference, but the least bad one).
> 
> Another option could be to encapsulate these function pointer casts as
> follows:
> #define REMOVE_NORETURN(x) (void(*)(void*))(x)
> This approach allows us to retain the noreturn attribute and the associated
> optimizations;
> note that the encapsulating macro will need to be deviated then.

I think that's OK.

Re: [PATCH] xen: address violation of MISRA C Rule 11.1

[Jan Beulich]

On 13.12.2024 15:02, Alessandro Zucchelli wrote:
> On 2024-12-13 11:08, Jan Beulich wrote:
>> On 13.12.2024 01:53, Stefano Stabellini wrote:
>>> On Thu, 12 Dec 2024, Jan Beulich wrote:
>>>> On 12.12.2024 03:27, Stefano Stabellini wrote:
>>>>> On Wed, 11 Dec 2024, Jan Beulich wrote:
>>>>>> On 11.12.2024 12:02, Alessandro Zucchelli wrote:
>>>>>>> Rule 11.1 states as following: "Conversions shall not be performed
>>>>>>> between a pointer to a function and any other type".
>>>>>>>
>>>>>>> Functions "__machine_restart" and "__machine_halt" in 
>>>>>>> "x86/shutdown.c"
>>>>>>> and "halt_this_cpu" in "arm/shutdown.c" are defined as noreturn
>>>>>>> functions and subsequently passed as parameters to function calls.
>>>>>>> This violates the rule in Clang, where the "noreturn" attribute is
>>>>>>> considered part of the function"s type.
>>>>>>
>>>>>> I'm unaware of build issues with Clang, hence can you clarify how 
>>>>>> Clang's
>>>>>> view comes into play here? In principle various attributes ought to 
>>>>>> be
>>>>>> part of a function's type; iirc that's also the case for gcc. Yet 
>>>>>> how
>>>>>> that matters to Eclair is still entirely unclear to me.
>>>>>>
>>>>>>> By removing the "noreturn"
>>>>>>> attribbute and replacing it with uses of the ASSERT_UNREACHABLE 
>>>>>>> macro,
>>>>>>> these violations are addressed.
>>>>>>
>>>>>> Papered over, I'd say. What about release builds, for example?
>>>>>>
>>>>>> Deleting the attribute also has a clear downside 
>>>>>> documentation-wise. If
>>>>>> we really mean to remove them from what the compiler gets to see, I 
>>>>>> think
>>>>>> we ought to still retain them in commented-out shape.
>>>>>
>>>>> Another option would be to #define noreturn to nothing for ECLAIR 
>>>>> builds ?
>>>>
>>>> That again would feel like papering over things. Plus I don't know if 
>>>> that's
>>>> an option at all.
>>>
>>> What is "papering over" and what is a "nice solution" is often up to 
>>> the
>>> personal opinions.
>>>
>>> From my point of view, Alessandro's patch doesn't make the code worse.
>>> The ASSERT_UNREACHABLE solution is OK. I do agree with you that it
>>> should not be required for us to remove "noreturn", but I don't think 
>>> we
>>> have used it consistently anyway across the Xen codebase.
>>> ASSERT_UNREACHABLE is also a form of documentation that the function
>>> does not return.
>>>
>>> In conclusion, I think all three options are acceptable:
>>> 1) this patch as is
>>> 2) this patch plus /* noreturn */ as a comment
>>> 3) #define noreturn to nothing just for ECLAIR builds
>>>
>>> I don't mind either way, maybe option 2) is the best compromise.
>>
>> The variant with least impact on what we currently have (generated code
>> wise) is 3), though, which hence would be my preference (well, not 
>> exactly
>> a preference, but the least bad one).
> 
> Another option could be to encapsulate these function pointer casts as 
> follows:
> #define REMOVE_NORETURN(x) (void(*)(void*))(x)
> This approach allows us to retain the noreturn attribute and the 
> associated optimizations;
> note that the encapsulating macro will need to be deviated then.

And then have one such macro for every attribute that may need zapping?
What if there are multiple? Any macro may do, yet which one to use would
be unclear. What if only some attributes need zapping, and some need
retaining?

Jan

Re: [PATCH] xen: address violation of MISRA C Rule 11.1

[Stefano Stabellini]

On Mon, 16 Dec 2024, Jan Beulich wrote:
> On 13.12.2024 15:02, Alessandro Zucchelli wrote:
> > On 2024-12-13 11:08, Jan Beulich wrote:
> >> On 13.12.2024 01:53, Stefano Stabellini wrote:
> >>> On Thu, 12 Dec 2024, Jan Beulich wrote:
> >>>> On 12.12.2024 03:27, Stefano Stabellini wrote:
> >>>>> On Wed, 11 Dec 2024, Jan Beulich wrote:
> >>>>>> On 11.12.2024 12:02, Alessandro Zucchelli wrote:
> >>>>>>> Rule 11.1 states as following: "Conversions shall not be performed
> >>>>>>> between a pointer to a function and any other type".
> >>>>>>>
> >>>>>>> Functions "__machine_restart" and "__machine_halt" in 
> >>>>>>> "x86/shutdown.c"
> >>>>>>> and "halt_this_cpu" in "arm/shutdown.c" are defined as noreturn
> >>>>>>> functions and subsequently passed as parameters to function calls.
> >>>>>>> This violates the rule in Clang, where the "noreturn" attribute is
> >>>>>>> considered part of the function"s type.
> >>>>>>
> >>>>>> I'm unaware of build issues with Clang, hence can you clarify how 
> >>>>>> Clang's
> >>>>>> view comes into play here? In principle various attributes ought to 
> >>>>>> be
> >>>>>> part of a function's type; iirc that's also the case for gcc. Yet 
> >>>>>> how
> >>>>>> that matters to Eclair is still entirely unclear to me.
> >>>>>>
> >>>>>>> By removing the "noreturn"
> >>>>>>> attribbute and replacing it with uses of the ASSERT_UNREACHABLE 
> >>>>>>> macro,
> >>>>>>> these violations are addressed.
> >>>>>>
> >>>>>> Papered over, I'd say. What about release builds, for example?
> >>>>>>
> >>>>>> Deleting the attribute also has a clear downside 
> >>>>>> documentation-wise. If
> >>>>>> we really mean to remove them from what the compiler gets to see, I 
> >>>>>> think
> >>>>>> we ought to still retain them in commented-out shape.
> >>>>>
> >>>>> Another option would be to #define noreturn to nothing for ECLAIR 
> >>>>> builds ?
> >>>>
> >>>> That again would feel like papering over things. Plus I don't know if 
> >>>> that's
> >>>> an option at all.
> >>>
> >>> What is "papering over" and what is a "nice solution" is often up to 
> >>> the
> >>> personal opinions.
> >>>
> >>> From my point of view, Alessandro's patch doesn't make the code worse.
> >>> The ASSERT_UNREACHABLE solution is OK. I do agree with you that it
> >>> should not be required for us to remove "noreturn", but I don't think 
> >>> we
> >>> have used it consistently anyway across the Xen codebase.
> >>> ASSERT_UNREACHABLE is also a form of documentation that the function
> >>> does not return.
> >>>
> >>> In conclusion, I think all three options are acceptable:
> >>> 1) this patch as is
> >>> 2) this patch plus /* noreturn */ as a comment
> >>> 3) #define noreturn to nothing just for ECLAIR builds
> >>>
> >>> I don't mind either way, maybe option 2) is the best compromise.
> >>
> >> The variant with least impact on what we currently have (generated code
> >> wise) is 3), though, which hence would be my preference (well, not 
> >> exactly
> >> a preference, but the least bad one).
> > 
> > Another option could be to encapsulate these function pointer casts as 
> > follows:
> > #define REMOVE_NORETURN(x) (void(*)(void*))(x)
> > This approach allows us to retain the noreturn attribute and the 
> > associated optimizations;
> > note that the encapsulating macro will need to be deviated then.
> 
> And then have one such macro for every attribute that may need zapping?
> What if there are multiple? Any macro may do, yet which one to use would
> be unclear. What if only some attributes need zapping, and some need
> retaining?

It is always a judgment call between addressing issues ad hoc and
developing generic solutions. In this case, since we are discussing only
one attribute, I do not think we should attempt to generalize it
further.