On Wed, Jul 12, 2023 at 8:36 AM Jan Beulich <jbeul...@suse.com> wrote: > > Certain fallback code can be made subject to DCE this way. Note that > CX16 has no compiler provided manifest constant, so CONFIG_* are used > there instead. Note also that we don't have cpu_has_movbe nor > cpu_has_lzcnt (aka cpu_has_abm). > > Signed-off-by: Jan Beulich <jbeul...@suse.com>
Reviewed-by: Jason Andryuk <jandr...@gmail.com> One thought below. > --- > Of course we could use IS_ENABLED(CONFIG_X86_64_V<n>) everywhere, but as > CX16 shows this isn't necessarily better than the #if/#else approach > based on compiler-provided manifest constants. While not really intended > to be used that way, it looks as if we could also use > IS_ENABLED(__POPCNT__) and alike. > > We could go further and also short-circuit SSE*, AVX and alike, which we > don't use outside of the emulator. > > --- a/xen/arch/x86/include/asm/cpufeature.h > +++ b/xen/arch/x86/include/asm/cpufeature.h > @@ -76,13 +76,19 @@ static inline bool boot_cpu_has(unsigned > #define cpu_has_eist boot_cpu_has(X86_FEATURE_EIST) > #define cpu_has_ssse3 boot_cpu_has(X86_FEATURE_SSSE3) > #define cpu_has_fma boot_cpu_has(X86_FEATURE_FMA) > -#define cpu_has_cx16 boot_cpu_has(X86_FEATURE_CX16) > +#define cpu_has_cx16 (IS_ENABLED(CONFIG_X86_64_V2) || \ > + IS_ENABLED(CONFIG_X86_64_V3) || \ > + boot_cpu_has(X86_FEATURE_CX16)) If you think there may be more ABI selections in the future, it might be better to express the "V$N" numerically and check >= 2. Or you can add a Kconfig CONFIG_X86_64_CX16 and select that as appropriate. But if there aren't going to be more of these, then this is fine. Regards, Jason
