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

[Borislav Petkov <bp@xxxxxxxxx>]

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@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(-)

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