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

[Jason Andryuk <jason.andryuk@xxxxxxx>]

On 2025-12-10 02:09, Jan Beulich wrote:
> On 09.12.2025 22:55, Jason Andryuk wrote:
> > 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?
> Both, as long as --sectname-subst is passed to the assembler. My patch adds
> that option to AFLAGS only, whereas for inline assembly it would need adding
> to CFLAGS. Did you perhaps overlook that?
Yes, you are correct - I had not changed CFLAGS.  With that updated, the 
%%S form creates sections:
.altinstructions..text.set_domain_state_info

Thanks,
Jason