svn commit: r344118 - head/sys/i386/include

[Alexey Dokuchaev]

On Thu, Feb 14, 2019 at 01:53:11PM +0000, Konstantin Belousov wrote:
> New Revision: 344118
> URL: https://svnweb.freebsd.org/changeset/base/344118
> 
> Log:
>   Provide userspace versions of do_cpuid() and cpuid_count() on i386.
>   
>   Some older compilers, when generating PIC code, cannot handle inline
>   asm that clobbers %ebx (because %ebx is used as the GOT offset
>   register).  Userspace versions avoid clobbering %ebx by saving it to
>   stack before executing the CPUID instruction.
>   
> ...
> +static __inline void
> +do_cpuid(u_int ax, u_int *p)
> +{
> +	__asm __volatile(
> +	    "pushl\t%%ebx\n\t"
> +	    "cpuid\n\t"
> +	    "movl\t%%ebx,%1\n\t"
> +	    "popl\t%%ebx"

Is there a reason to prefer pushl+movl+popl instead of movl+xchgl?

    "movl %%ebx, %1\n\t"
    "cpuid\n\t"
    "xchgl %%ebx, %1"

./danfe