Re: [Qemu-devel] [PATCH 3/4] exec, memory: Call to xen_modified_memory.

[Anthony PERARD]

On 17/07/12 14:37, Avi Kivity wrote:
> On 07/17/2012 04:30 PM, Anthony PERARD wrote:
> > This patch add some calls to xen_modified_memory to notify Xen about dirtybits
> > during migration.
> >
> > diff --git a/exec.c b/exec.c
> > index c9fa17d..9f7a4f7 100644
> > --- a/exec.c
> > +++ b/exec.c
> > @@ -3438,6 +3438,7 @@ void cpu_physical_memory_rw(target_phys_addr_t addr, 
> > uint8_t *buf,
> >                       cpu_physical_memory_set_dirty_flags(
> >                           addr1, (0xff & ~CODE_DIRTY_FLAG));
> >                   }
> > +                xen_modified_memory(addr1, TARGET_PAGE_SIZE);
> >                   qemu_put_ram_ptr(ptr);
> >               }
> >           } else {
>
> This is pretty ugly.  An alternative is to set up a periodic bitmap scan
> that looks at the qemu dirty bitmap and calls xen_modified_memory() for
> dirty page ranges, and clears the bitmap for the next pass.  Is it workable?

I don't think a periodic scan can do anything useful, unfortunately.

> (is xen_modified_memory a hypercall, or does it maintain an in-memory
> structure?)

It's an hypercall. The function do something (call the hypercall) only 
during migration, otherwise it return immediately.

--
Anthony PERARD