Re: [Qemu-devel] [PATCH] Permit -mem-path without sync mmu

[Marcelo Tosatti <mtosatti@redhat.com>]

On Fri, Aug 05, 2011 at 08:16:42AM +0200, Jan Kiszka wrote:
> On 2011-08-05 06:02, David Gibson wrote:
> > At present, an explicit test disallows use of -mem-path when kvm is enabled
> > but KVM_CAP_SYNC_MMU is not set.  In particular, this prevents the user
> > from using hugetlbfs to back the guest memory.
> > 
> > I can see no reason for this check, and when I asked about it previously,
> > the only theory offered was that this was a limitation of the very early
> > days of kvm which only happened to match the SYNC_MMU flag by accident.
> > 
> > This patch, therefore, removes the check.  This is of particular use to
> > us on POWER, where we haven't yet implement SYNC_MMU, but where backing
> > the guest with hugepages is possible, and in fact mandatory (for now).
> > 
> > Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> > ---
> >  exec.c |    5 -----
> >  1 files changed, 0 insertions(+), 5 deletions(-)
> > 
> > diff --git a/exec.c b/exec.c
> > index 476b507..041637c 100644
> > --- a/exec.c
> > +++ b/exec.c
> > @@ -2818,11 +2818,6 @@ static void *file_ram_alloc(RAMBlock *block,
> >          return NULL;
> >      }
> >  
> > -    if (kvm_enabled() && !kvm_has_sync_mmu()) {
> > -        fprintf(stderr, "host lacks kvm mmu notifiers, -mem-path unsupported\n");
> > -        return NULL;
> > -    }
> > -
> >      if (asprintf(&filename, "%s/qemu_back_mem.XXXXXX", path) == -1) {
> >          return NULL;
> >      }
> 
> This is nothing trivial, see ce9a92411d in qemu-kvm or
> http://thread.gmane.org/gmane.comp.emulators.kvm.devel/27380. And it
> should rather target uq/master. CCing Avi, Marcelo, and the kvm list.
> 
> Jan

Yes, the check cannot be removed because there is the possibility of
corruption using hugepages without mmu notifiers (described in the 
archived message above).

Why are mmu notifiers not implemented for PPC again?