Re: [PATCH v3 07/15] x86/alternative: support "not feature" and ALTERNATIVE_TERNARY

Borislav Petkov
2021-01-07T19:09:34Z

On Thu, Dec 17, 2020 at 10:31:25AM +0100, Juergen Gross wrote:
> Instead of only supporting to modify instructions when a specific
> feature is set, support doing so for the case a feature is not set.
>
> As today a feature is specified using a 16 bit quantity and the highest
> feature number in use is around 600, using a negated feature number for
> specifying the inverted case seems to be appropriate.
>
> ALTERNATIVE "default_instr", "patched_instr", ~FEATURE_NR
>
> will start with "default_instr" and patch that with "patched_instr" in
> case FEATURE_NR is not set.
>
> Using that add ALTERNATIVE_TERNARY:
>
> ALTERNATIVE_TERNARY "default_instr", FEATURE_NR,
> "feature_on_instr", "feature_off_instr"
>
> which will start with "default_instr" and at patch time will, depending
> on FEATURE_NR being set or not, patch that with either
> "feature_on_instr" or "feature_off_instr".

How about an even simpler one (only build-tested):

---
diff --git a/arch/x86/include/asm/alternative-asm.h b/arch/x86/include/asm/alternative-asm.h
index 464034db299f..d52b423d3cab 100644
--- a/arch/x86/include/asm/alternative-asm.h
+++ b/arch/x86/include/asm/alternative-asm.h
@@ -109,6 +109,9 @@
.popsection
.endm

+#define ALTERNATIVE_TERNARY(oldinstr, feature, newinstr1, newinstr2) \
+ ALTERNATIVE_2 oldinstr, newinstr1, feature, newinstr2, X86_FEATURE_TERNARY
+
#endif /* __ASSEMBLY__ */

#endif /* _ASM_X86_ALTERNATIVE_ASM_H */
diff --git a/arch/x86/include/asm/alternative.h b/arch/x86/include/asm/alternative.h
index 13adca37c99a..f170cbe89539 100644
--- a/arch/x86/include/asm/alternative.h
+++ b/arch/x86/include/asm/alternative.h
@@ -175,6 +175,9 @@ static inline int alternatives_text_reserved(void *start, void *end)
ALTINSTR_REPLACEMENT(newinstr2, feature2, 2) \
".popsection\n"

+#define ALTERNATIVE_TERNARY(oldinstr, feature, newinstr1, newinstr2) \
+ ALTERNATIVE_2(oldinstr, newinstr1, feature, newinstr2, X86_FEATURE_TERNARY)
+
#define ALTERNATIVE_3(oldinsn, newinsn1, feat1, newinsn2, feat2, newinsn3, feat3) \
OLDINSTR_3(oldinsn, 1, 2, 3) \
".pushsection .altinstructions,\"a\"\n" \
@@ -206,6 +209,9 @@ static inline int alternatives_text_reserved(void *start, void *end)
#define alternative_2(oldinstr, newinstr1, feature1, newinstr2, feature2) \
asm_inline volatile(ALTERNATIVE_2(oldinstr, newinstr1, feature1, newinstr2, feature2) ::: "memory")

+#define alternative_ternary(oldinstr, feature, newinstr1, newinstr2) \
+ asm_inline volatile(ALTERNATIVE_TERNARY(oldinstr, feature, newinstr1, newinstr2) ::: "memory")
+
/*
* Alternative inline assembly with input.
*
diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
index 84b887825f12..cc634db0b91f 100644
--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -108,7 +108,7 @@
#define X86_FEATURE_EXTD_APICID ( 3*32+26) /* Extended APICID (8 bits) */
#define X86_FEATURE_AMD_DCM ( 3*32+27) /* AMD multi-node processor */
#define X86_FEATURE_APERFMPERF ( 3*32+28) /* P-State hardware coordination feedback capability (APERF/MPERF MSRs) */
-/* free ( 3*32+29) */
+#define X86_FEATURE_TERNARY ( 3*32+29) /* "" Synthetic bit for ALTERNATIVE_TERNARY() */
#define X86_FEATURE_NONSTOP_TSC_S3 ( 3*32+30) /* TSC doesn't stop in S3 state */
#define X86_FEATURE_TSC_KNOWN_FREQ ( 3*32+31) /* TSC has known frequency */

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index 8d778e46725d..2cb29d4d8dd9 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -393,7 +393,7 @@ void __init_or_module noinline apply_alternatives(struct alt_instr *start,
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)) {
+ if (!boot_cpu_has(a->cpuid) && (a->cpuid != X86_FEATURE_TERNARY)) {
if (a->padlen > 1)
optimize_nops(a, instr);

--
Regards/Gruss,
Boris.