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

[Stefano Stabellini]

On Tue, 17 Jul 2012, Avi Kivity wrote:
> On 07/17/2012 04:59 PM, Anthony PERARD wrote:
> >>
> >> 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.
> 
> Why not?

I vaguely remember that we used to have a bitmap years ago, but, aside from
making the code much more complicated, it caused blue screens on
intensive disk accesses.


> >> (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.
> 
> I see.  I guess it isn't expensive for you because there isn't much dma
> done by qemu usually with xen (unlike kvm where pv block devices are
> implemented in qemu).
> 
> How about pushing the call into cpu_physical_memory_set_dirty_flags()?
> Would that reduce the number of call sites?

Pushing the calls to cpu_physical_memory_set_dirty_flags and
cpu_physical_memory_set_dirty_range would make the code much nicer.
However being these functions in exec-obsolete.h, are they at risk of
removal?