Re: [Qemu-devel] env fields as TCG variables

[Edgar E. Iglesias]

On Tue, Sep 16, 2008 at 09:41:56PM +0200, Edgar E. Iglesias wrote:
> On Tue, Sep 16, 2008 at 10:31:01PM +0300, Blue Swirl wrote:
> > On 9/16/08, Edgar E. Iglesias <address@hidden> wrote:
> > > On Tue, Sep 16, 2008 at 09:37:41PM +0300, Blue Swirl wrote:
> > >  > I wonder if the following bug scenario is possible.
> > >  >
> > >  > Some of the env fields are accessible as TCG variables, like cpu_fsr
> > >  > for Sparc floating point status register (%fsr), stored in the env
> > >  > structure field env->fsr. When cpu_fsr is used in the generated code,
> > >  > a host CPU register is allocated for this task. But env->fsr can be
> > >  > modified directly in op_helper.c code. Isn't it then possible that the
> > >  > register (which stores cpu_fsr) and the structure field in memory
> > >  > (env->fsr) get out of sync?
> > >  >
> > >  > If this is true, no code outside the generated code should ever modify
> > >  > or even read env fields that are also available as TCG variables. Or
> > >  > do I miss something?
> > >
> > >
> > > Hello,
> > >
> > >  IIRC:
> > >  TCG writebacks before calling helpers. There used to be a bug with
> > >  qemu_ld/qemu_st like the one you described but it was fixed a long
> > >  time ago. I ran into that one with CRIS and avoided it for a while
> > >  by calling a dummy helper before load/stores.
> > 
> > OK, this solves the main part of the problem. But what about code
> > outside helpers, for example interrupt handling?
> 
> Hi Again, Im not sure but if I understand correctly:
> Interrupts are only taken between TB's and TCG writebacks globals at the end 
> of TBs.
> MMU faults work by writebacking globals before loads/stores.

Sorry that was only for D faults. I faults are only taken between TB's AFAICT, 
so those are OK.
Unless you do like CRIS and fetch across page boundaries within the same TB, 
but that's my bug :)

Best regards