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

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Date:   Wed, 10 Mar 2021 07:07:05 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Juergen Gross <jgross@...e.com>
Cc:     xen-devel@...ts.xenproject.org, x86@...nel.org,
        linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [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@...e.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.

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