[PATCH v3 07/15] x86/alternative: support "not feature" and ALTERNATIVE_TERNARY

Juergen Gross
2020-12-17T09:33:37Z

Instead of only supporting to modify instructions when a specific
feature is set, support doing so for the case a feature is not set.

As today a feature is specified using a 16 bit quantity and the highest
feature number in use is around 600, using a negated feature number for
specifying the inverted case seems to be appropriate.

ALTERNATIVE "default_instr", "patched_instr", ~FEATURE_NR

will start with "default_instr" and patch that with "patched_instr" in
case FEATURE_NR is not set.

Using that add ALTERNATIVE_TERNARY:

ALTERNATIVE_TERNARY "default_instr", FEATURE_NR,
"feature_on_instr", "feature_off_instr"

which will start with "default_instr" and at patch time will, depending
on FEATURE_NR being set or not, patch that with either
"feature_on_instr" or "feature_off_instr".

Signed-off-by: Juergen Gross <jgross@suse.com>
---
V3:
- new patch
---
arch/x86/include/asm/alternative-asm.h | 3 +++
arch/x86/include/asm/alternative.h | 7 +++++++
arch/x86/kernel/alternative.c | 17 ++++++++++++-----
3 files changed, 22 insertions(+), 5 deletions(-)

diff --git a/arch/x86/include/asm/alternative-asm.h b/arch/x86/include/asm/alternative-asm.h
index 464034db299f..b6989995fddf 100644
--- a/arch/x86/include/asm/alternative-asm.h
+++ b/arch/x86/include/asm/alternative-asm.h
@@ -109,6 +109,9 @@
.popsection
.endm

+#define ALTERNATIVE_TERNARY(oldinstr, feature, newinstr1, newinstr2) \
+ ALTERNATIVE_2 oldinstr, newinstr1, feature, newinstr2, ~(feature)
+
#endif /* __ASSEMBLY__ */

#endif /* _ASM_X86_ALTERNATIVE_ASM_H */
diff --git a/arch/x86/include/asm/alternative.h b/arch/x86/include/asm/alternative.h
index 13adca37c99a..a0f8f33609aa 100644
--- a/arch/x86/include/asm/alternative.h
+++ b/arch/x86/include/asm/alternative.h
@@ -59,6 +59,7 @@ struct alt_instr {
s32 instr_offset; /* original instruction */
s32 repl_offset; /* offset to replacement instruction */
u16 cpuid; /* cpuid bit set for replacement */
+#define ALT_INSTR_CPUID_INV 0x8000 /* patch if ~cpuid bit is NOT set */
u8 instrlen; /* length of original instruction */
u8 replacementlen; /* length of new instruction */
u8 padlen; /* length of build-time padding */
@@ -175,6 +176,9 @@ static inline int alternatives_text_reserved(void *start, void *end)
ALTINSTR_REPLACEMENT(newinstr2, feature2, 2) \
".popsection\n"

+#define ALTERNATIVE_TERNARY(oldinstr, feature, newinstr1, newinstr2) \
+ ALTERNATIVE_2(oldinstr, newinstr1, feature, newinstr2, ~(feature))
+
#define ALTERNATIVE_3(oldinsn, newinsn1, feat1, newinsn2, feat2, newinsn3, feat3) \
OLDINSTR_3(oldinsn, 1, 2, 3) \
".pushsection .altinstructions,\"a\"\n" \
@@ -206,6 +210,9 @@ static inline int alternatives_text_reserved(void *start, void *end)
#define alternative_2(oldinstr, newinstr1, feature1, newinstr2, feature2) \
asm_inline volatile(ALTERNATIVE_2(oldinstr, newinstr1, feature1, newinstr2, feature2) ::: "memory")

+#define alternative_ternary(oldinstr, feature, newinstr1, newinstr2) \
+ asm_inline volatile(ALTERNATIVE_TERNARY(oldinstr, feature, newinstr1, newinstr2) ::: "memory")
+
/*
* Alternative inline assembly with input.
*
diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index 8d778e46725d..0a904fb2678b 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -388,21 +388,28 @@ void __init_or_module noinline apply_alternatives(struct alt_instr *start,
*/
for (a = start; a < end; a++) {
int insn_buff_sz = 0;
+ u16 feature;
+ bool not_feature;

instr = (u8 *)&a->instr_offset + a->instr_offset;
replacement = (u8 *)&a->repl_offset + a->repl_offset;
+ feature = a->cpuid;
+ not_feature = feature & ALT_INSTR_CPUID_INV;
+ if (not_feature)
+ feature = ~feature;
BUG_ON(a->instrlen > sizeof(insn_buff));
- BUG_ON(a->cpuid >= (NCAPINTS + NBUGINTS) * 32);
- if (!boot_cpu_has(a->cpuid)) {
+ BUG_ON(feature >= (NCAPINTS + NBUGINTS) * 32);
+ if (!!boot_cpu_has(feature) == not_feature) {
if (a->padlen > 1)
optimize_nops(a, instr);

continue;
}

- DPRINTK("feat: %d*32+%d, old: (%pS (%px) len: %d), repl: (%px, len: %d), pad: %d",
- a->cpuid >> 5,
- a->cpuid & 0x1f,
+ DPRINTK("feat: %s%d*32+%d, old: (%pS (%px) len: %d), repl: (%px, len: %d), pad: %d",
+ not_feature ? "~" : "",
+ feature >> 5,
+ feature & 0x1f,
instr, instr, a->instrlen,
replacement, a->replacementlen, a->padlen);

--
2.26.2