On 25/09/2025 11:48 am, Jan Beulich wrote:
> 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 <[email protected]>
>
> --- 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" )
I don't understand. How is this related to putting the guard in place?
> --- 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
Again - why are we making a no-op change for the sake of it?
> #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 )
Not a bug in this change, but the pre-existing use of positional labels
is something I was expecting not to introduce at all seeing as we
started cleanly with named labels.
The jmp wants to be:
"jmp %l" #label
to cope with the fact it's a macro parameter too.
>
> 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");
> }
As the comment says, this is already an abuse of the macro for a purpose
it wasn't intended for.
Now requiring an extra "nop" parameter to get the abuse to compile is
too far. It can turn into a plain ALTERNATIVE(), and then both disappear.
~Andrew
