[PATCH 2/2] x86: guard synthetic feature and bug enumerators

[Jan Beulich <jbeulich@xxxxxxxx>]

While adding new enumerators one may overlook the (rare) need to bump
X86_NR_{SYNTH,BUG}. Guard against that happening by adding respective
checking. The use of BUILD_BUG_ON_ZERO(), however, entails a number of
other changes, as the expansion may not appear in the assembly produced.
Furthermore inputs to file-scope asm() are only supported in gcc15 (or
newer).

No difference in generated code (debug info, however, grows quite a bit).

An implication from the changes is that users of the alternatives patching
macros may not use unnamed asm() input operands anymore, as the "injected"
new operands would break numbering expectations.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/include/asm/alternative.h
+++ b/xen/arch/x86/include/asm/alternative.h
@@ -70,12 +70,12 @@ extern void alternative_instructions(voi
                     alt_repl_len(n2)) "-" alt_orig_len)
 
 #define ALTINSTR_ENTRY(feature, num)                                    \
-        " .if (" STR(feature & ~ALT_FLAG_NOT) ") >= " STR(NCAPINTS * 32) "\n" \
+        " .if (%c" #feature " & " STR(~ALT_FLAG_NOT) ") >= " STR(NCAPINTS * 
32) "\n" \
         " .error \"alternative feature outside of featureset range\"\n" \
         " .endif\n"                                                     \
         " .long .LXEN%=_orig_s - .\n"             /* label           */ \
         " .long " alt_repl_s(num)" - .\n"         /* new instruction */ \
-        " .word " STR(feature) "\n"               /* feature bit     */ \
+        " .word %c" #feature "\n"                 /* feature bit     */ \
         " .byte " alt_orig_len "\n"               /* source len      */ \
         " .byte " alt_repl_len(num) "\n"          /* replacement len */ \
         " .byte " alt_pad_len "\n"                /* padding len     */ \
@@ -127,14 +127,14 @@ extern void alternative_instructions(voi
  */
 #define alternative(oldinstr, newinstr, feature)                        \
     asm_inline volatile (                                               \
-        ALTERNATIVE(oldinstr, newinstr, feature)                        \
-        ::: "memory" )
+        ALTERNATIVE(oldinstr, newinstr, [feat])                         \
+        :: [feat] "i" (feature) : "memory" )
 
 #define alternative_2(oldinstr, newinstr1, feature1, newinstr2, feature2) \
     asm_inline volatile (                                               \
-        ALTERNATIVE_2(oldinstr, newinstr1, feature1,                    \
-                      newinstr2, feature2)                              \
-        ::: "memory" )
+        ALTERNATIVE_2(oldinstr, newinstr1, [feat1],                     \
+                      newinstr2, [feat2])                               \
+        ::  [feat1] "i" (feature1), [feat2] "i" (feature2) : "memory" )
 
 /*
  * Alternative inline assembly with input.
@@ -148,14 +148,14 @@ extern void alternative_instructions(voi
  */
 #define alternative_input(oldinstr, newinstr, feature, input...)        \
     asm_inline volatile (                                               \
-        ALTERNATIVE(oldinstr, newinstr, feature)                        \
-        :: input )
+        ALTERNATIVE(oldinstr, newinstr, [feat])                         \
+        :: [feat] "i" (feature), ## input  )
 
 /* Like alternative_input, but with a single output argument */
 #define alternative_io(oldinstr, newinstr, feature, output, input...)   \
     asm_inline volatile (                                               \
-        ALTERNATIVE(oldinstr, newinstr, feature)                        \
-        : output : input )
+        ALTERNATIVE(oldinstr, newinstr, [feat])                         \
+        : output : [feat] "i" (feature), ## input )
 
 /*
  * This is similar to alternative_io. But it has two features and
@@ -168,9 +168,9 @@ extern void alternative_instructions(voi
 #define alternative_io_2(oldinstr, newinstr1, feature1, newinstr2,      \
                          feature2, output, input...)                    \
     asm_inline volatile (                                               \
-        ALTERNATIVE_2(oldinstr, newinstr1, feature1,                    \
-                      newinstr2, feature2)                              \
-        : output : input )
+        ALTERNATIVE_2(oldinstr, newinstr1, [feat1],                     \
+                      newinstr2, [feat2])                               \
+        : output : [feat1] "i" (feature1), [feat2] "i" (feature2), ## input )
 
 /* Use this macro(s) if you need more than one output parameter. */
 #define ASM_OUTPUT2(a...) a
--- a/xen/arch/x86/include/asm/cpufeatures.h
+++ b/xen/arch/x86/include/asm/cpufeatures.h
@@ -6,9 +6,16 @@
 /* Number of capability words covered by the featureset words. */
 #define FSCAPINTS FEATURESET_NR_ENTRIES
 
+#if !defined(__ASSEMBLY__) && __GNUC__ >= 15
+#include <xen/macros.h>
+#define X86_CHECK_FEAT_NR(x, n) BUILD_BUG_ON_ZERO((x) / 32 >= X86_NR_ ## n)
+#else
+#define X86_CHECK_FEAT_NR(x, n) 0
+#endif
+
 /* Synthetic words follow the featureset words. */
 #define X86_NR_SYNTH 2
-#define X86_SYNTH(x) (FSCAPINTS * 32 + (x))
+#define X86_SYNTH(x) (FSCAPINTS * 32 + (x) + X86_CHECK_FEAT_NR(x, SYNTH))
 
 /* Synthetic features */
 XEN_CPUFEATURE(CONSTANT_TSC,      X86_SYNTH( 0)) /* TSC ticks at a constant 
rate */
@@ -47,7 +54,8 @@ XEN_CPUFEATURE(XEN_REP_MOVSB,     X86_SY
 
 /* Bug words follow the synthetic words. */
 #define X86_NR_BUG 1
-#define X86_BUG(x) ((FSCAPINTS + X86_NR_SYNTH) * 32 + (x))
+#define X86_BUG(x) ((FSCAPINTS + X86_NR_SYNTH) * 32 + (x) + \
+                    X86_CHECK_FEAT_NR(x, BUG))
 
 #define X86_BUG_FPU_PTRS          X86_BUG( 0) /* (F)X{SAVE,RSTOR} doesn't 
save/restore FOP/FIP/FDP. */
 #define X86_BUG_NULL_SEG          X86_BUG( 1) /* NULL-ing a selector preserves 
the base and limit. */
--- a/xen/arch/x86/include/asm/cpufeatureset.h
+++ b/xen/arch/x86/include/asm/cpufeatureset.h
@@ -12,8 +12,13 @@ enum {
 };
 #undef XEN_CPUFEATURE
 
+#if __GNUC__ >= 15
+#define XEN_CPUFEATURE(name, value) asm (".equ X86_FEATURE_" #name ", %c0" \
+                                         :: "i" (X86_FEATURE_##name));
+#else
 #define XEN_CPUFEATURE(name, value) asm (".equ X86_FEATURE_" #name ", " \
                                          __stringify(value));
+#endif
 #include <public/arch-x86/cpufeatureset.h>
 #include <asm/cpufeatures.h>
 
--- a/xen/arch/x86/include/asm/pdx.h
+++ b/xen/arch/x86/include/asm/pdx.h
@@ -13,9 +13,9 @@
     asm_inline goto (                               \
         ALTERNATIVE(                                \
             "",                                     \
-            "jmp %l0",                              \
-            ALT_NOT(X86_FEATURE_PDX_COMPRESSION))   \
-        : : : : label )
+            "jmp %l1",                              \
+            [feat])                                 \
+        : : [feat] "i" (ALT_NOT(X86_FEATURE_PDX_COMPRESSION)) : : label )
 
 static inline unsigned long pfn_to_pdx(unsigned long pfn)
 {
--- a/xen/arch/x86/include/asm/spec_ctrl.h
+++ b/xen/arch/x86/include/asm/spec_ctrl.h
@@ -73,7 +73,7 @@ static always_inline void spec_ctrl_new_
 
     /* (ab)use alternative_input() to specify clobbers. */
     alternative_input("", "DO_OVERWRITE_RSB xu=%=", X86_BUG_IBPB_NO_RET,
-                      : "rax", "rcx");
+                      "i" (0) : "rax", "rcx");
 }
 
 extern int8_t opt_ibpb_ctxt_switch;
@@ -163,7 +163,7 @@ static always_inline void __spec_ctrl_en
      * (ab)use alternative_input() to specify clobbers.
      */
     alternative_input("", "DO_OVERWRITE_RSB xu=%=", X86_FEATURE_SC_RSB_IDLE,
-                      : "rax", "rcx");
+                      "i" (0) : "rax", "rcx");
 }
 
 /* WARNING! `ret`, `call *`, `jmp *` not safe after this call. */
--- a/xen/arch/x86/include/asm/tsc.h
+++ b/xen/arch/x86/include/asm/tsc.h
@@ -29,8 +29,7 @@ static inline uint64_t rdtsc_ordered(voi
     alternative_io_2("lfence; rdtsc",
                      "mfence; rdtsc", X86_FEATURE_MFENCE_RDTSC,
                      "rdtscp",        X86_FEATURE_RDTSCP,
-                     ASM_OUTPUT2("=a" (low), "=d" (high), "=c" (aux)),
-                     /* no inputs */);
+                     ASM_OUTPUT2("=a" (low), "=d" (high), "=c" (aux)));
 
     return (high << 32) | low;
 }