Re: [Qemu-devel] [PATCH 2/2] Revived GUEST_BASE support for usermode emulation targets.

[Mika Westerberg]

On Apr 10, 2009, at 2:22 AM, malc wrote:

> On Thu, 9 Apr 2009, address@hidden wrote:
>
> > From: Riku Voipio <address@hidden>
> >
> > From: Mika Westerberg <address@hidden>
> >
> > - Now GUEST_BASE is dynamic and can be set from command line.
> > - Qemu checks /proc/sys/vm/mmap_min_addr and sets GUEST_BASE
> >  if needed.
> > - Code generation supports GUEST_BASE for i386 and x86_64 hosts.
> >
> > Signed-off-by: Riku Voipio <address@hidden>
> [..snip..]
>
> > diff --git a/tcg/i386/tcg-target.c b/tcg/i386/tcg-target.c
> > index e0fd434..7cb51b5 100644
> > --- a/tcg/i386/tcg-target.c
> > +++ b/tcg/i386/tcg-target.c
> > @@ -560,6 +560,12 @@ static void tcg_out_qemu_ld(TCGContext *s,  
> > const TCGArg *args,
> >     /* add x(r1), r0 */
> >     tcg_out_modrm_offset(s, 0x03, r0, r1, offsetof(CPUTLBEntry,  
> > addend) -
> >                          offsetof(CPUTLBEntry, addr_read));
> > +#elif defined(CONFIG_USE_GUEST_BASE)
> > +    /*
> > +     * Add guest_base to all loads.
> > +     */
> > +    tcg_out_mov(s, r0, addr_reg);    /* movl addr_reg, r0 */
> > +    tcg_out_addi(s, r0, GUEST_BASE); /* addl $GUEST_BASE, r0 */
>
> I think 'lea' is warranted here, and likewise in _st and x86_64 case.

Can you be more specific about that? Do you mean that we should change  
MOV into
LEA?

I think that in this case, as we are just moving address (pointer)  
from one register to another, it should be ok.  For example gcc  
generates from this:

static char *
f(char *p)
{
        return (p + 65536);
}

following code:

00000000 <f>:
   0:   55                      push   %ebp
   1:   89 e5                   mov    %esp,%ebp
   3:   8b 45 08                mov    0x8(%ebp),%eax
   6:   05 00 00 01 00          add    $0x10000,%eax
   b:   c9                      leave
   c:   c3                      ret

Or am I missing something?

Thanks,
MW