Re: [PATCH v6 18/19] migration/multifd: Stop changing the packet on recv side

[Peter Xu]

On Tue, Aug 27, 2024 at 03:45:11PM -0300, Fabiano Rosas wrote:
> Peter Xu <peterx@redhat.com> writes:
> 
> > On Tue, Aug 27, 2024 at 02:46:05PM -0300, Fabiano Rosas wrote:
> >> @@ -254,12 +250,10 @@ int multifd_ram_unfill_packet(MultiFDRecvParams *p, 
> >> Error **errp)
> >>          return 0;
> >>      }
> >>  
> >> -    /* make sure that ramblock is 0 terminated */
> >> -    packet->ramblock[255] = 0;
> >> -    p->block = qemu_ram_block_by_name(packet->ramblock);
> >> +    ramblock_name = g_strndup(packet->ramblock, 255);
> >
> > I understand we want to move to a const*, however this introduces a 256B
> > allocation per multifd packet, which we definitely want to avoid.. I wonder
> > whether that's worthwhile just to make it const. :-(
> >
> > I don't worry too much on the const* and vars pointed being abused /
> > updated when without it - the packet struct is pretty much limited only to
> > be referenced in this unfill function, and then we will do the load based
> > on MultiFDRecvParams* later anyway.  So personally I'd rather lose the
> > const* v.s. one allocation.
> >
> > Or we could also sanity check byte 255 to be '\0' (which, AFAIU, should
> > always be the case..), then we can get both benefits.
> 
> We can't because it breaks compat. Previous QEMUs didn't zero the
> packet.

Ouch!

Then.. shall we still try to avoid the allocation?

-- 
Peter Xu