[PATCH -tip v2 0/3] x86/callthunks: Fix and unify call thunk assembly snippets

[Uros Bizjak]

Currently INCREMENT_CALL_DEPTH and thunk debug macros explicitly
define %gs: segment register prefix for their percpu variables.
This is not compatible with !CONFIG_SMP, which requires non-prefixed
percpu variables.

Contrary to alternatives, relocations are currently not supported in
call thunk templates.  Due to unsupported relocations, two variants of
INCREMENT_CALL_DEPTH macro are needed, ASM_ prefixed that allows
relocations and non-prefixed version that allows only absolute
addresses.

The following patch series fixes above issues by

a) Making PER_CPU_VAR macro from percpu.h also available without
__ASSEMBLY__, so it can be used in 'asm' statements to conditionally
use %gs: segment register prefix, depending on CONFIG_SMP.

b) Re-using existing infrastructure from alternative.c to allow
%rip-relative relocations when copying call thunk template from its
storage location.

c) Fixing call thunks debug macros to use PER_CPU_VAR macro from
percpu.h to conditionally use %gs: segment register prefix, depending
on CONFIG_SMP.

d) Unifying ASM_ prefixed assembly macros with their non-prefixed
variants. With support of %rip-relative relocations in place, call
thunk templates allow %rip-relative addressing, so unified assembly
snippet can be used everywhere.

v2:
- Make PER_CPU_VAR macro available in 'asm' statements instead of
  moving 'asm' statements to *.S assembly files.
- Re-use existing relocation infrastructure from alternative.c

Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>

Uros Bizjak (3):
  x86/percpu: Define PER_CPU_VAR macro also for !__ASSEMBLY__
  x86/callthunks: Handle %rip-relative relocations in call thunk
    template
  x86/callthunks: Fix and unify call thunks assembly snippets

 arch/x86/include/asm/nospec-branch.h | 23 ++++++--------------
 arch/x86/include/asm/percpu.h        |  5 +++++
 arch/x86/include/asm/text-patching.h |  2 ++
 arch/x86/kernel/alternative.c        |  3 +--
 arch/x86/kernel/callthunks.c         | 32 ++++++++++++++++++++++------
 5 files changed, 40 insertions(+), 25 deletions(-)

-- 
2.41.0