Stefano Stabellini
2012-Jul-17 18:36 UTC
Re: [Qemu-devel] [PATCH 3/4] exec, memory: Call to xen_modified_memory.
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?