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SubjectRe: [PATCH v6 04/12] x86/alternative: support not-feature
On Tue, Mar 09, 2021 at 02:48:05PM +0100, Juergen Gross wrote:
> 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@suse.com>
> ---
> 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(-)

LGTM, minor touchups I'd do ontop:

---

diff --git a/arch/x86/include/asm/alternative.h b/arch/x86/include/asm/alternative.h
index 89889618ae01..fd205cdcfbad 100644
--- a/arch/x86/include/asm/alternative.h
+++ b/arch/x86/include/asm/alternative.h
@@ -55,18 +55,18 @@
".long 999b - .\n\t" \
".popsection\n\t"

+#define ALTINSTR_FLAG_INV (1 << 15)
+#define ALT_NOT(feat) ((feat) | ALTINSTR_FLAG_INV)
+
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 d8e669a1546f..133b549dc091 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -397,9 +397,10 @@ void __init_or_module noinline apply_alternatives(struct alt_instr *start,
BUG_ON(feature >= (NCAPINTS + NBUGINTS) * 32);

/*
- * Drop out if either:
- * - feature not available, but required, or
- * - feature available, but NOT required
+ * 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)
--
Regards/Gruss,
Boris.

https://people.kernel.org/tglx/notes-about-netiquette

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