Re: [Qemu-devel] Re: [PATCH 21/26] Implement TCE translation for sPAPR VIO

[Anthony Liguori]

On 03/16/2011 03:05 PM, Benjamin Herrenschmidt wrote:
> On Wed, 2011-03-16 at 17:03 +0100, Alexander Graf wrote:
> > > +int spapr_tce_dma_zero(VIOsPAPRDevice *dev, uint64_t taddr,
> > uint32_t size)
> > > +{
> > > +    uint8_t *zeroes;
> > > +
> > > +#ifdef DEBUG_TCE
> > > +    fprintf(stderr, "spapr_tce_dma_zero taddr=0x%llx size=0x%x\n",
> > > +            (unsigned long long)taddr, size);
> > > +#endif
> > > +
> > > +    /* FIXME: do this better... */
> > > +    zeroes = alloca(size);
> > > +    memset(zeroes, 0, size);
> > You sure that zeroes is still alive during the call? If I were a
> > compiler, I'd probably optimize the return away so that it'd end up
> > being a simple branch to spapr_tce_dma_write - coincidentally
> > invalidating the stack that zeroes is on.
> Ugh ? How would this ever be legal for a compiler to do that ?

Yeah, the compiler can't do that.  The return of alloca() is valid as 
long as the stack frame is valid.  Inlining doesn't change that.
Regards,

Anthony Liguori

> Ben.