[PATCH v7 05/14] x86/alternative: support not-feature

[Juergen Gross <jgross@xxxxxxxx>]

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)
V7:
- minor tweaks: move defines up, comment wording (Boris Petkov)
---
 arch/x86/include/asm/alternative.h |  3 +++
 arch/x86/kernel/alternative.c      | 20 +++++++++++++++-----
 2 files changed, 18 insertions(+), 5 deletions(-)

diff --git a/arch/x86/include/asm/alternative.h 
b/arch/x86/include/asm/alternative.h
index 53f295f41c34..649e56f70889 100644
--- a/arch/x86/include/asm/alternative.h
+++ b/arch/x86/include/asm/alternative.h
@@ -6,6 +6,9 @@
 #include <linux/stringify.h>
 #include <asm/asm.h>
 
+#define ALTINSTR_FLAG_INV      (1 << 15)
+#define ALT_NOT(feat)          ((feat) | ALTINSTR_FLAG_INV)
+
 #ifndef __ASSEMBLY__
 
 #include <linux/stddef.h>
diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index 8d778e46725d..133b549dc091 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -388,21 +388,31 @@ 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);
+
+               /*
+                * Patch if either:
+                * - feature is present
+                * - feature not present but ALTINSTR_FLAG_INV is set to mean,
+                *   patch if feature is *NOT* present.
+                */
+               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