[PATCH] xen: always consider '/' as a division in assembly

[PATCH] xen: always consider '/' as a division in assembly

[Roger Pau Monne]

GNU assembler will consider '/' as the start of comment marker on some
platforms.  This is incorrect with Xen's usage, which does use '/' in
assembly files as a mathematical operator.

The behavior of the assembler can be altered by passing the --divide
option; unconditionally pass this option when available to force the
expected behavior.

Reported-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
---
 xen/Makefile | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/xen/Makefile b/xen/Makefile
index 31352d4cd260..eee0262933a7 100644
--- a/xen/Makefile
+++ b/xen/Makefile
@@ -405,6 +405,11 @@ $(call cc-option-add,CFLAGS,CC,-Winit-self)
 CFLAGS += -pipe -D__XEN__ -include $(srctree)/include/xen/config.h
 CFLAGS-$(CONFIG_DEBUG_INFO) += -g
 
+# The GNU assembler will interpret '/' as a comment start marker instead of a
+# divide on some platforms.  Pass --divide when when available to signal '/' is
+# always used as an operator in assembly.
+$(call cc-option-add,CFLAGS,CC,-Wa$$(comma)--divide)
+
 ifneq ($(CONFIG_CC_IS_CLANG),y)
 # Clang doesn't understand this command line argument, and doesn't appear to
 # have a suitable alternative.  The resulting compiled binary does function,
-- 
2.51.0

Re: [PATCH] xen: always consider '/' as a division in assembly

[Jan Beulich]

On 10.02.2026 16:11, Roger Pau Monne wrote:
> GNU assembler will consider '/' as the start of comment marker on some
> platforms.  This is incorrect with Xen's usage, which does use '/' in
> assembly files as a mathematical operator.
> 
> The behavior of the assembler can be altered by passing the --divide
> option; unconditionally pass this option when available to force the
> expected behavior.

I'm fine with this in principle, but I wonder: What about Clang? If it's
properly compatible, it ought to also take '/' as a comment char for
those same targets (in particular for the plain "x86_64-elf" one).
According to godbolt it can't deal with -Wa,--divide, yet there I also
can't control what exact target the toolchain supports (i.e. this may be
only a weak indication of lack of support / compatibility).

> --- a/xen/Makefile
> +++ b/xen/Makefile
> @@ -405,6 +405,11 @@ $(call cc-option-add,CFLAGS,CC,-Winit-self)
>  CFLAGS += -pipe -D__XEN__ -include $(srctree)/include/xen/config.h
>  CFLAGS-$(CONFIG_DEBUG_INFO) += -g
>  
> +# The GNU assembler will interpret '/' as a comment start marker instead of a
> +# divide on some platforms.

Could I talk you into s/on some platforms/for some ELF targets/ ?

A more fundamental question is: Do we really mean to support (allow)
building with arbitrary-target toolchains? There are other subtle
differences, which may be hard to evaluate as to them possibly affecting
the Xen build.

>  Pass --divide when when available to signal '/' is
> +# always used as an operator in assembly.
> +$(call cc-option-add,CFLAGS,CC,-Wa$$(comma)--divide)

I don't think this should be done here: --divide is an x86-specific
option.

Jan

Re: [PATCH] xen: always consider '/' as a division in assembly

[Roger Pau Monné]

On Tue, Feb 10, 2026 at 04:27:33PM +0100, Jan Beulich wrote:
> On 10.02.2026 16:11, Roger Pau Monne wrote:
> > GNU assembler will consider '/' as the start of comment marker on some
> > platforms.  This is incorrect with Xen's usage, which does use '/' in
> > assembly files as a mathematical operator.
> > 
> > The behavior of the assembler can be altered by passing the --divide
> > option; unconditionally pass this option when available to force the
> > expected behavior.
> 
> I'm fine with this in principle, but I wonder: What about Clang? If it's
> properly compatible, it ought to also take '/' as a comment char for
> those same targets (in particular for the plain "x86_64-elf" one).
> According to godbolt it can't deal with -Wa,--divide, yet there I also
> can't control what exact target the toolchain supports (i.e. this may be
> only a weak indication of lack of support / compatibility).

Hm, I'm unsure I can get such a Clang build to properly test it.  Note
the checking for the presence of the option would also be done in
Clang.  I would like to think if Clang integrated assembler has this
behavior the option to select our expected behavior will also be
--divide.  I think that's the best I can do ATM.

> > --- a/xen/Makefile
> > +++ b/xen/Makefile
> > @@ -405,6 +405,11 @@ $(call cc-option-add,CFLAGS,CC,-Winit-self)
> >  CFLAGS += -pipe -D__XEN__ -include $(srctree)/include/xen/config.h
> >  CFLAGS-$(CONFIG_DEBUG_INFO) += -g
> >  
> > +# The GNU assembler will interpret '/' as a comment start marker instead of a
> > +# divide on some platforms.
> 
> Could I talk you into s/on some platforms/for some ELF targets/ ?

Yes, that's fine IMO.

> A more fundamental question is: Do we really mean to support (allow)
> building with arbitrary-target toolchains? There are other subtle
> differences, which may be hard to evaluate as to them possibly affecting
> the Xen build.

Hm, TBH I wasn't aware of such subtle and annoying differences until
today, so I'm not sure what to reply here.  I haven't tested the x86
image yet, as I'm still attempting to resolve some non-POSIX options
usage in check-endbr.sh.

We might want to state which ELF targets we do explicitly support for
production usage.  TBH I doubt anyone would use a Darwin build for
production.

> >  Pass --divide when when available to signal '/' is
> > +# always used as an operator in assembly.
> > +$(call cc-option-add,CFLAGS,CC,-Wa$$(comma)--divide)
> 
> I don't think this should be done here: --divide is an x86-specific
> option.

Oh, that wasn't clear from the output of as.  I can make it
x86-specific then.  However, is there any chance of this behavior
escaping outside of x86, and hence would we like to ensure this
behavior on all possible arches?

Or is it possible that other arches re-use the --divide option for
some other functionality?

Thanks, Roger.

Re: [PATCH] xen: always consider '/' as a division in assembly

[Jan Beulich]

On 10.02.2026 16:53, Roger Pau Monné wrote:
> On Tue, Feb 10, 2026 at 04:27:33PM +0100, Jan Beulich wrote:
>> On 10.02.2026 16:11, Roger Pau Monne wrote:
>>>  Pass --divide when when available to signal '/' is
>>> +# always used as an operator in assembly.
>>> +$(call cc-option-add,CFLAGS,CC,-Wa$$(comma)--divide)
>>
>> I don't think this should be done here: --divide is an x86-specific
>> option.
> 
> Oh, that wasn't clear from the output of as.  I can make it
> x86-specific then.  However, is there any chance of this behavior
> escaping outside of x86, and hence would we like to ensure this
> behavior on all possible arches?
> 
> Or is it possible that other arches re-use the --divide option for
> some other functionality?

It is equally possible for that to happen (if some target needs to
support '/' as a comment char) as it is for such a target to then use
an entirely different command line option. Furthermore, --divide may
mean something else on another arch.

Jan

Re: [PATCH] xen: always consider '/' as a division in assembly

[Bertrand Marquis]

Hi Roger,

> On 10 Feb 2026, at 16:11, Roger Pau Monne <roger.pau@citrix.com> wrote:
> 
> GNU assembler will consider '/' as the start of comment marker on some
> platforms.  This is incorrect with Xen's usage, which does use '/' in
> assembly files as a mathematical operator.
> 
> The behavior of the assembler can be altered by passing the --divide
> option; unconditionally pass this option when available to force the
> expected behavior.

This is solving the issue and I can build for arm with what and further
for x86 on mac os but it fails on check-endbr.sh:

/bin/sh ../tools/check-endbr.sh xen-syms
stat: illegal option -- c

Cheers
Bertrand

> 
> Reported-by: Jan Beulich <jbeulich@suse.com>
> Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
> ---
> xen/Makefile | 5 +++++
> 1 file changed, 5 insertions(+)
> 
> diff --git a/xen/Makefile b/xen/Makefile
> index 31352d4cd260..eee0262933a7 100644
> --- a/xen/Makefile
> +++ b/xen/Makefile
> @@ -405,6 +405,11 @@ $(call cc-option-add,CFLAGS,CC,-Winit-self)
> CFLAGS += -pipe -D__XEN__ -include $(srctree)/include/xen/config.h
> CFLAGS-$(CONFIG_DEBUG_INFO) += -g
> 
> +# The GNU assembler will interpret '/' as a comment start marker instead of a
> +# divide on some platforms.  Pass --divide when when available to signal '/' is
> +# always used as an operator in assembly.
> +$(call cc-option-add,CFLAGS,CC,-Wa$$(comma)--divide)
> +
> ifneq ($(CONFIG_CC_IS_CLANG),y)
> # Clang doesn't understand this command line argument, and doesn't appear to
> # have a suitable alternative.  The resulting compiled binary does function,
> -- 
> 2.51.0
> 

Re: [PATCH] xen: always consider '/' as a division in assembly

[Roger Pau Monné]

On Tue, Feb 10, 2026 at 03:44:33PM +0000, Bertrand Marquis wrote:
> Hi Roger,
> 
> > On 10 Feb 2026, at 16:11, Roger Pau Monne <roger.pau@citrix.com> wrote:
> > 
> > GNU assembler will consider '/' as the start of comment marker on some
> > platforms.  This is incorrect with Xen's usage, which does use '/' in
> > assembly files as a mathematical operator.
> > 
> > The behavior of the assembler can be altered by passing the --divide
> > option; unconditionally pass this option when available to force the
> > expected behavior.
> 
> This is solving the issue and I can build for arm with what and further
> for x86 on mac os but it fails on check-endbr.sh:
> 
> /bin/sh ../tools/check-endbr.sh xen-syms
> stat: illegal option -- c

Yeah, I know.  I've fixed the stat issue, but there's a further one
about an illegal bit sequence in grep.  I'm looking into that.

Thanks, Roger.

Re: [PATCH] xen: always consider '/' as a division in assembly

[Bertrand Marquis]

Hi Roger

> On 10 Feb 2026, at 16:55, Roger Pau Monné <roger.pau@citrix.com> wrote:
> 
> On Tue, Feb 10, 2026 at 03:44:33PM +0000, Bertrand Marquis wrote:
>> Hi Roger,
>> 
>>> On 10 Feb 2026, at 16:11, Roger Pau Monne <roger.pau@citrix.com> wrote:
>>> 
>>> GNU assembler will consider '/' as the start of comment marker on some
>>> platforms.  This is incorrect with Xen's usage, which does use '/' in
>>> assembly files as a mathematical operator.
>>> 
>>> The behavior of the assembler can be altered by passing the --divide
>>> option; unconditionally pass this option when available to force the
>>> expected behavior.
>> 
>> This is solving the issue and I can build for arm with what and further
>> for x86 on mac os but it fails on check-endbr.sh:
>> 
>> /bin/sh ../tools/check-endbr.sh xen-syms
>> stat: illegal option -- c
> 
> Yeah, I know.  I've fixed the stat issue, but there's a further one
> about an illegal bit sequence in grep.  I'm looking into that.

LC_ALL=C grep is fixing that one on my side

with that and stat -c replaced with wc -c < xx I can fully compile x86 on Mac os.

Cheers
Bertrand

> 
> Thanks, Roger.

Re: [PATCH] xen: always consider '/' as a division in assembly

[Roger Pau Monné]

On Tue, Feb 10, 2026 at 03:57:17PM +0000, Bertrand Marquis wrote:
> Hi Roger
> 
> > On 10 Feb 2026, at 16:55, Roger Pau Monné <roger.pau@citrix.com> wrote:
> > 
> > On Tue, Feb 10, 2026 at 03:44:33PM +0000, Bertrand Marquis wrote:
> >> Hi Roger,
> >> 
> >>> On 10 Feb 2026, at 16:11, Roger Pau Monne <roger.pau@citrix.com> wrote:
> >>> 
> >>> GNU assembler will consider '/' as the start of comment marker on some
> >>> platforms.  This is incorrect with Xen's usage, which does use '/' in
> >>> assembly files as a mathematical operator.
> >>> 
> >>> The behavior of the assembler can be altered by passing the --divide
> >>> option; unconditionally pass this option when available to force the
> >>> expected behavior.
> >> 
> >> This is solving the issue and I can build for arm with what and further
> >> for x86 on mac os but it fails on check-endbr.sh:
> >> 
> >> /bin/sh ../tools/check-endbr.sh xen-syms
> >> stat: illegal option -- c
> > 
> > Yeah, I know.  I've fixed the stat issue, but there's a further one
> > about an illegal bit sequence in grep.  I'm looking into that.
> 
> LC_ALL=C grep is fixing that one on my side

I haven't tested that one yet, but yes, might be a way to solve.
Would you like to send the fix for grep?

> with that and stat -c replaced with wc -c < xx I can fully compile x86 on Mac os.

Yeah, the problem with using wc is that you have to read the file.  I
prefer $(($(du <file> |cut -w -f1) << 9)) because that avoids reading
the actual data to count.

Thanks, Roger.

Re: [PATCH] xen: always consider '/' as a division in assembly

[Jan Beulich]

On 10.02.2026 17:09, Roger Pau Monné wrote:
> On Tue, Feb 10, 2026 at 03:57:17PM +0000, Bertrand Marquis wrote:
>> Hi Roger
>>
>>> On 10 Feb 2026, at 16:55, Roger Pau Monné <roger.pau@citrix.com> wrote:
>>>
>>> On Tue, Feb 10, 2026 at 03:44:33PM +0000, Bertrand Marquis wrote:
>>>> Hi Roger,
>>>>
>>>>> On 10 Feb 2026, at 16:11, Roger Pau Monne <roger.pau@citrix.com> wrote:
>>>>>
>>>>> GNU assembler will consider '/' as the start of comment marker on some
>>>>> platforms.  This is incorrect with Xen's usage, which does use '/' in
>>>>> assembly files as a mathematical operator.
>>>>>
>>>>> The behavior of the assembler can be altered by passing the --divide
>>>>> option; unconditionally pass this option when available to force the
>>>>> expected behavior.
>>>>
>>>> This is solving the issue and I can build for arm with what and further
>>>> for x86 on mac os but it fails on check-endbr.sh:
>>>>
>>>> /bin/sh ../tools/check-endbr.sh xen-syms
>>>> stat: illegal option -- c
>>>
>>> Yeah, I know.  I've fixed the stat issue, but there's a further one
>>> about an illegal bit sequence in grep.  I'm looking into that.
>>
>> LC_ALL=C grep is fixing that one on my side
> 
> I haven't tested that one yet, but yes, might be a way to solve.
> Would you like to send the fix for grep?
> 
>> with that and stat -c replaced with wc -c < xx I can fully compile x86 on Mac os.
> 
> Yeah, the problem with using wc is that you have to read the file.  I
> prefer $(($(du <file> |cut -w -f1) << 9)) because that avoids reading
> the actual data to count.

But shifting by hardcoded 9 doesn't look very portable. The size not being
exact may be okay for the purposes here, but wouldn't it then be better to
use (standardized) ls output?

Jan

Re: [PATCH] xen: always consider '/' as a division in assembly

[Bertrand Marquis]

Hi,

> On 10 Feb 2026, at 17:15, Jan Beulich <jbeulich@suse.com> wrote:
> 
> On 10.02.2026 17:09, Roger Pau Monné wrote:
>> On Tue, Feb 10, 2026 at 03:57:17PM +0000, Bertrand Marquis wrote:
>>> Hi Roger
>>> 
>>>> On 10 Feb 2026, at 16:55, Roger Pau Monné <roger.pau@citrix.com> wrote:
>>>> 
>>>> On Tue, Feb 10, 2026 at 03:44:33PM +0000, Bertrand Marquis wrote:
>>>>> Hi Roger,
>>>>> 
>>>>>> On 10 Feb 2026, at 16:11, Roger Pau Monne <roger.pau@citrix.com> wrote:
>>>>>> 
>>>>>> GNU assembler will consider '/' as the start of comment marker on some
>>>>>> platforms.  This is incorrect with Xen's usage, which does use '/' in
>>>>>> assembly files as a mathematical operator.
>>>>>> 
>>>>>> The behavior of the assembler can be altered by passing the --divide
>>>>>> option; unconditionally pass this option when available to force the
>>>>>> expected behavior.
>>>>> 
>>>>> This is solving the issue and I can build for arm with what and further
>>>>> for x86 on mac os but it fails on check-endbr.sh:
>>>>> 
>>>>> /bin/sh ../tools/check-endbr.sh xen-syms
>>>>> stat: illegal option -- c
>>>> 
>>>> Yeah, I know.  I've fixed the stat issue, but there's a further one
>>>> about an illegal bit sequence in grep.  I'm looking into that.
>>> 
>>> LC_ALL=C grep is fixing that one on my side
>> 
>> I haven't tested that one yet, but yes, might be a way to solve.
>> Would you like to send the fix for grep?
>> 
>>> with that and stat -c replaced with wc -c < xx I can fully compile x86 on Mac os.
>> 
>> Yeah, the problem with using wc is that you have to read the file.  I
>> prefer $(($(du <file> |cut -w -f1) << 9)) because that avoids reading
>> the actual data to count.
> 
> But shifting by hardcoded 9 doesn't look very portable. The size not being
> exact may be okay for the purposes here, but wouldn't it then be better to
> use (standardized) ls output?

Suggesting this:
wc -c < "$TEXT_BIN"

Bertrand

Re: [PATCH] xen: always consider '/' as a division in assembly

[Bertrand Marquis]

Hi Roger,

> On 10 Feb 2026, at 17:09, Roger Pau Monné <roger.pau@citrix.com> wrote:
> 
> On Tue, Feb 10, 2026 at 03:57:17PM +0000, Bertrand Marquis wrote:
>> Hi Roger
>> 
>>> On 10 Feb 2026, at 16:55, Roger Pau Monné <roger.pau@citrix.com> wrote:
>>> 
>>> On Tue, Feb 10, 2026 at 03:44:33PM +0000, Bertrand Marquis wrote:
>>>> Hi Roger,
>>>> 
>>>>> On 10 Feb 2026, at 16:11, Roger Pau Monne <roger.pau@citrix.com> wrote:
>>>>> 
>>>>> GNU assembler will consider '/' as the start of comment marker on some
>>>>> platforms.  This is incorrect with Xen's usage, which does use '/' in
>>>>> assembly files as a mathematical operator.
>>>>> 
>>>>> The behavior of the assembler can be altered by passing the --divide
>>>>> option; unconditionally pass this option when available to force the
>>>>> expected behavior.
>>>> 
>>>> This is solving the issue and I can build for arm with what and further
>>>> for x86 on mac os but it fails on check-endbr.sh:
>>>> 
>>>> /bin/sh ../tools/check-endbr.sh xen-syms
>>>> stat: illegal option -- c
>>> 
>>> Yeah, I know.  I've fixed the stat issue, but there's a further one
>>> about an illegal bit sequence in grep.  I'm looking into that.
>> 
>> LC_ALL=C grep is fixing that one on my side
> 
> I haven't tested that one yet, but yes, might be a way to solve.
> Would you like to send the fix for grep?
> 
>> with that and stat -c replaced with wc -c < xx I can fully compile x86 on Mac os.
> 
> Yeah, the problem with using wc is that you have to read the file.  I
> prefer $(($(du <file> |cut -w -f1) << 9)) because that avoids reading
> the actual data to count.

Both fixes should be included in the same patch.

Will submit one once we agree on the solution for stat -c.
I suggested:
wc -c < "$TEXT_BIN"

Tell me if that works for you.

Cheers
Bertrand

> 
> Thanks, Roger.