Re: [Qemu-devel] [PATCH] Revived GUEST_BASE support for usermode emulation targets [v3]

[Paul Brook]

On Tuesday 05 May 2009, Riku Voipio wrote:
> On Tue, May 05, 2009 at 02:53:31PM +0100, Paul Brook wrote:
> > On Tuesday 05 May 2009, Riku Voipio wrote:
> > > +++ b/tcg/x86_64/tcg-target.c
> > > @@ -616,15 +616,15 @@ static void tcg_out_qemu_ld(TCGContext *s, const
> > >          /* movzbl */
> > > -        tcg_out_modrm_offset(s, 0xb6 | P_EXT, data_reg, r0, 0);
> > > +        tcg_out_modrm_offset(s, 0xb6 | P_EXT, data_reg, r0,
> > > GUEST_BASE);
> >
> > This breaks when GUEST_BASE is large.
>
> usually we don't set it to very large, just high enough to overcome
> mmap_min_addr limits.

I'd expect a common use would be to move a 32-bit guest to an entirely empty 
range of address space. Chances are this is going to be >4G.

> > 32-bit x86 is probably ok because GUEST_BASE is always a 32-bit value.
> > This is not true for x86-64. Incidentally tcg_out_modrm doesn't seem to
> > check this, and silently generates broken code.
>
> Should we rather check for a mazimum size on guest_base or revert to the
> lea -based version?

I expect LEA has the same bug. We definitely need to be able to handle large 
offsets on 64-bit hosts, though obviously we want to avoid the overhead when 
the offset is small.

Paul