[PATCH v6 04/12] x86/alternative: support not-feature

[Juergen Gross]

Add support for alternative patching for the case a feature is not
present on the current cpu.

For users of ALTERNATIVE() and friends an inverted feature is specified
by applying the ALT_NOT() macro to it, e.g.:

ALTERNATIVE(old, new, ALT_NOT(feature))

Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
---
V5:
- split off from next patch
- reworked to use flag byte (Boris Petkov)
V6:
- rework again to not use flag byte (Boris Petkov)
---
 arch/x86/include/asm/alternative-asm.h |  3 +++
 arch/x86/include/asm/alternative.h     |  3 +++
 arch/x86/kernel/alternative.c          | 19 ++++++++++++++-----
 3 files changed, 20 insertions(+), 5 deletions(-)

diff --git a/arch/x86/include/asm/alternative-asm.h b/arch/x86/include/asm/alternative-asm.h
index 464034db299f..3965daf0460e 100644
--- a/arch/x86/include/asm/alternative-asm.h
+++ b/arch/x86/include/asm/alternative-asm.h
@@ -6,6 +6,9 @@
 
 #include <asm/asm.h>
 
+#define ALTINSTR_FLAG_INV	(1 << 15)
+#define ALT_NOT(feat)		((feat) | ALTINSTR_FLAG_INV)
+
 #ifdef CONFIG_SMP
 	.macro LOCK_PREFIX
 672:	lock
diff --git a/arch/x86/include/asm/alternative.h b/arch/x86/include/asm/alternative.h
index 5753fb2ac489..89889618ae01 100644
--- a/arch/x86/include/asm/alternative.h
+++ b/arch/x86/include/asm/alternative.h
@@ -59,11 +59,14 @@ struct alt_instr {
 	s32 instr_offset;	/* original instruction */
 	s32 repl_offset;	/* offset to replacement instruction */
 	u16 cpuid;		/* cpuid bit set for replacement */
+#define ALTINSTR_FLAG_INV (1 << 15)
 	u8  instrlen;		/* length of original instruction */
 	u8  replacementlen;	/* length of new instruction */
 	u8  padlen;		/* length of build-time padding */
 } __packed;
 
+#define ALT_NOT(feat)	((feat) | ALTINSTR_FLAG_INV)
+
 /*
  * Debug flag that can be tested to see whether alternative
  * instructions were patched in already:
diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index 8d778e46725d..d8e669a1546f 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -388,21 +388,30 @@ void __init_or_module noinline apply_alternatives(struct alt_instr *start,
 	 */
 	for (a = start; a < end; a++) {
 		int insn_buff_sz = 0;
+		/* Mask away "NOT" flag bit for feature to test. */
+		u16 feature = a->cpuid & ~ALTINSTR_FLAG_INV;
 
 		instr = (u8 *)&a->instr_offset + a->instr_offset;
 		replacement = (u8 *)&a->repl_offset + a->repl_offset;
 		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);
+
+		/*
+		 * Drop out if either:
+		 * - feature not available, but required, or
+		 * - feature available, but NOT required
+		 */
+		if (!boot_cpu_has(feature) == !(a->cpuid & ALTINSTR_FLAG_INV)) {
 			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",
+			(a->cpuid & ALTINSTR_FLAG_INV) ? "!" : "",
+			feature >> 5,
+			feature & 0x1f,
 			instr, instr, a->instrlen,
 			replacement, a->replacementlen, a->padlen);
 
-- 
2.26.2