Re: [Qemu-devel] [PATCH] Permit zero-sized qemu_malloc() & friends

[Avi Kivity]

On 12/06/2009 06:52 PM, Ian Molton wrote:
> Markus Armbruster wrote:
>
>    
> >      p = malloc(n * sizeof(struct foo);
> >      if (n&&  !p)
> >          exit_no_mem();
> >      for (i = 0; i<  n; i++)
> >          compute_one(p, i);
> >
> > With qemu_malloc(), the error handling moves into qemu_malloc():
> >
> >      p = qemu_malloc(n * sizeof(struct foo);
> >      for (i = 0; i<  n; i++)
> >          compute_one(p, i);
> >      
> And you lose the ability to fail gracefully...
>    

We never had it.  Suppose p is allocated in response to an SCSI register 
write, and we allocate a scatter-gather list.  What do you do if you OOM?

1) Introduce an error path that works synchronously off the stack and so 
does not need to allocate memory
2) Don't allocate in the first place, always read guest memory and 
transform it for sglists, preallocate everything else
3) Have a per-thread emergency pool, stall the vcpu until all memory is 
returned to the emergency pool

all of these options are pretty horrible, either to the code or to the 
guest, for something that never happens.

--
error compiling committee.c: too many arguments to function