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Date: Fri, 15 Feb 2019 12:36:44 +0200
From: Konstantin Belousov <kostikbel@gmail.com>
To: Alexey Dokuchaev <danfe@freebsd.org>
Cc: src-committers@freebsd.org, svn-src-all@freebsd.org,
 svn-src-head@freebsd.org
Subject: Re: svn commit: r344118 - head/sys/i386/include
Message-ID: <20190215103644.GN24863@kib.kiev.ua>
References: <201902141353.x1EDrB0Z076223@repo.freebsd.org>
 <20190215071604.GA89653@FreeBSD.org>
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On Fri, Feb 15, 2019 at 07:16:04AM +0000, Alexey Dokuchaev wrote:
> 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"

xchgl seems to be slower even in registers format (where no implicit
lock is used).  If you can demonstrate that your fragment is better in
some microbenchmark, I can change it.  But also note that its use is not
on the critical path.