[Xen-devel] [PATCH v2 4/5] alternative/x86/arm32: Align altinstructions (and altinstr_replacement) sections.

[Konrad Rzeszutek Wilk <konrad@xxxxxxxxxx>]

This is very similar to 137c59b9ff3f7a214f03b52d9c00a0a02374af1f
"bug/x86/arm: Align bug_frames sections."

On ARM and on x86 the C and assembler macros don't include
any alignment information - hence they end up being the default
byte granularity.

On ARM32 it is paramount that the alignment is word-size (4)
otherwise if one tries to use (uint32_t*) access (such
as livepatch ELF relocations) we get a Data Abort.

Enforcing .altinstructions (and also .altinstr_replacement for
completness) to have the proper alignment across all
architectures and in both C and x86 makes them all the same.

On x86 the bloat-o-meter detects that with this change the file shrinks:
add/remove: 1/0 grow/shrink: 0/2 up/down: 156/-367 (-211)
function                                     old     new   delta
get_page_from_gfn                              -     156    +156
do_mmu_update                               4578    4569      -9
do_mmuext_op                                5604    5246    -358
Total: Before=3170439, After=3170228, chg -0.01%

While on ARM 32/64:
add/remove: 0/0 grow/shrink: 0/0 up/down: 0/0 (0)
function                                     old     new   delta
Total: Before=822563, After=822563, chg +0.00%

Signed-off-by: Konrad Rzeszutek Wilk <konrad@xxxxxxxxxx>
---
v2: First version
---
 xen/include/asm-arm/alternative.h | 4 ++++
 xen/include/asm-x86/alternative.h | 2 ++
 2 files changed, 6 insertions(+)

diff --git a/xen/include/asm-arm/alternative.h 
b/xen/include/asm-arm/alternative.h
index 6cc9d0dc5f..cd1373fdd5 100644
--- a/xen/include/asm-arm/alternative.h
+++ b/xen/include/asm-arm/alternative.h
@@ -54,9 +54,11 @@ int apply_alternatives(const struct alt_instr *start, const 
struct alt_instr *en
        oldinstr "\n"                                                   \
        "662:\n"                                                        \
        ".pushsection .altinstructions,\"a\"\n"                         \
+       ".p2align 2\n"                                                  \
        ALTINSTR_ENTRY(feature)                                         \
        ".popsection\n"                                                 \
        ".pushsection .altinstr_replacement, \"a\"\n"                   \
+       ".p2align 2\n"                                                  \
        "663:\n\t"                                                      \
        newinstr "\n"                                                   \
        "664:\n\t"                                                      \
@@ -84,6 +86,7 @@ int apply_alternatives(const struct alt_instr *start, const 
struct alt_instr *en
        .if \enable
 661:   \insn1
 662:   .pushsection .altinstructions, "a"
+       .p2align 2
        altinstruction_entry 661b, 663f, \cap, 662b-661b, 664f-663f
        .popsection
        .pushsection .altinstr_replacement, "ax"
@@ -103,6 +106,7 @@ int apply_alternatives(const struct alt_instr *start, const 
struct alt_instr *en
 .macro alternative_if_not cap, enable = 1
        .if \enable
        .pushsection .altinstructions, "a"
+       .p2align 2
        altinstruction_entry 661f, 663f, \cap, 662f-661f, 664f-663f
        .popsection
 661:
diff --git a/xen/include/asm-x86/alternative.h 
b/xen/include/asm-x86/alternative.h
index db4f08e0e7..e667ccfbdb 100644
--- a/xen/include/asm-x86/alternative.h
+++ b/xen/include/asm-x86/alternative.h
@@ -56,10 +56,12 @@ extern void alternative_instructions(void);
 
 #define ALTERNATIVE_N(newinstr, feature, number)       \
        ".pushsection .altinstructions,\"a\"\n"         \
+       ".p2align 2\n"                                  \
        ALTINSTR_ENTRY(feature, number)                 \
        ".section .discard,\"a\",@progbits\n"           \
        DISCARD_ENTRY(number)                           \
        ".section .altinstr_replacement, \"ax\"\n"      \
+       ".p2align 2\n"                                  \
        ALTINSTR_REPLACEMENT(newinstr, feature, number) \
        ".popsection\n"
 
-- 
2.13.3


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