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

On 07.01.21 20:08, Borislav Petkov wrote:
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.


                       "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 
index 464034db299f..d52b423d3cab 100644
--- a/arch/x86/include/asm/alternative-asm.h
+++ b/arch/x86/include/asm/alternative-asm.h
@@ -109,6 +109,9 @@
+#define ALTERNATIVE_TERNARY(oldinstr, feature, newinstr1, newinstr2) \
+       ALTERNATIVE_2 oldinstr, newinstr1, feature, newinstr2, 
  #endif  /*  __ASSEMBLY__  */
#endif /* _ASM_X86_ALTERNATIVE_ASM_H */
diff --git a/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)                    \
+#define ALTERNATIVE_TERNARY(oldinstr, feature, newinstr1, newinstr2) \
+       ALTERNATIVE_2(oldinstr, newinstr1, feature, newinstr2, 
  #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 
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 

In fact this should rather be named "X86_FEATURE_TRUE", as this is its

And I think I can define it to the value 0xffff instead of using a
"real" bit for it.


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