Re: [PATCH] RFC: xen/x86: Enable --gc-sections

[Jason Andryuk <jason.andryuk@xxxxxxx>]

On 2025-12-08 03:46, Jan Beulich wrote:
> On 05.12.2025 23:40, Andrew Cooper wrote:
> > On 05/12/2025 10:28 pm, Jason Andryuk wrote:
> > > When linking to create xen-syms, add --gc-sections to garbage collect
> > > unused stuff.  Relies on CONFIG_CC_SPLIT_SECTIONS
> > >
> > > We need to add KEEP() to the linker script in assorted places to retain
> > > appropriate data - especially the arrays created therein.
> > >
> > > Something is off though.  In a test where memory_add() is unreachable,
> > > it is still included.  I'm not sure, but I am wondering if it's the
> > > alternatives somehow keeping a reference to it.
> > Yes, .altinstructions contains relocations against the origin patch
> > site, which will cause it to appear to be referenced. The same will be
> > happening with a bunch of other sections.
> We will need to derive helper section names from base section ones. See
> e.g. HAVE_AS_SECTNAME_SUBST as introduced by "common: honor
> CONFIG_CC_SPLIT_SECTIONS also for assembly functions", controlling the
> use of the --sectname-subst asssembler option, to in turn be able to use
> %S in section names (available from gas 2.26 onwards).
I tried to add your patch and change ALTERNATIVE to:
".pushsection .altinstructions.%S, \"a\", @progbits\n"
but it fails to build.  One example:
./include/xen/compiler.h:62:21: error: invalid 'asm': operand number 
missing after %-letter
   62 | # define asm_inline asm __inline
      |                     ^~~
./arch/x86/include/asm/pdx.h:13:5: note: in expansion of macro ‘asm_inline’
   13 |     asm_inline goto (                               \
      |     ^~~~~~~~~~
./arch/x86/include/asm/pdx.h:22:5: note: in expansion of macro 
‘PDX_ASM_GOTO’
   22 |     PDX_ASM_GOTO(skip);
      |     ^~~~~~~~~~~~

".pushsection .altinstructions.%%S, \"a\", @progbits\n" ends up creating 
section ".altinstructions.%S" which doesn't helpful.
Is %S expected to work with inline asm, or only standalone?

Regards,
Jason