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Date: Thu, 19 Mar 2015 14:27:55 -0400
From: Kevin O'Connor <kevin@koconnor.net>
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Subject: Re: [Qemu-devel] [PATCH 6/6] qga: RFC: guest-side retrieval of
 fw_cfg file
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To: Laszlo Ersek <lersek@redhat.com>
Cc: matt.fleming@intel.com, mdroth@linux.vnet.ibm.com, rjones@redhat.com, jordan.l.justen@intel.com, "Gabriel L. Somlo" <somlo@cmu.edu>, gleb@cloudius-systems.com, qemu-devel@nongnu.org, gsomlo@gmail.com, kraxel@redhat.com, pbonzini@redhat.com

On Tue, Mar 17, 2015 at 01:38:06PM +0100, Laszlo Ersek wrote:
> On 03/16/15 15:15, Gabriel L. Somlo wrote:
> >   1. I can't for the life of me figure out how to stop gcc -O2
> >      from assuming the if() test below is ALWAYS FALSE, and thus
> >      optimizing it out completely. For now I've forced -O0 on
> >      the entire function, but for some reason fw_cfg_read(&fcfile, ...)
> >      does not appear to count as potentially modifying fcfile...
[...]
> > +static void
> > +fw_cfg_read(void *buf, int len)
> > +{
> > +    insb(PORT_FW_CFG_DATA, buf, len);
> > +}
[...]
> I think fw_cfg_read() is inlined under -O2, and the insb() from that
> function is somehow confusing gcc.
> 
> From "/usr/include/sys/io.h", on my RHEL-7.1 laptop:
> 
> static __inline void
> insb (unsigned short int __port, void *__addr, unsigned long int __count)
> {
>   __asm__ __volatile__ ("cld ; rep ; insb":"=D" (__addr), "=c" (__count)
>                         :"d" (__port), "0" (__addr), "1" (__count));
> }

My read of this is that gcc knows it must emit the instruction, and it
knows that __addr and __count can change.  But, it doesn't know that
the memory at *__addr can change.  I'd see if a barrier() fixes it.

See the section on "clobber" at:

https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html

In particular:

  You can use a trick to avoid this if the size of the memory being
  accessed is known at compile time. For example, if accessing ten bytes
  of a string, use a memory input like:

  {"m"( ({ struct { char x[10]; } *p = (void *)ptr ; *p; }) )}.

-Kevin