Re: [Qemu-devel] [RFC 0/7] Rework vhost memory region updates

[Igor Mammedov]

On Thu, 30 Nov 2017 12:08:06 +0000
"Dr. David Alan Gilbert" <address@hidden> wrote:

> * Igor Mammedov (address@hidden) wrote:
> > On Wed, 29 Nov 2017 18:50:19 +0000
> > "Dr. David Alan Gilbert (git)" <address@hidden> wrote:
> >   
> > > From: "Dr. David Alan Gilbert" <address@hidden>
> > > 
> > > Hi,
> > >   This is an experimental set that reworks the way the vhost
> > > code handles changes in physical address space layout that
> > > came from a discussion with Igor.  
> > Thanks for looking into it.
> > 
> >    
> > > Instead of updating and trying to merge sections of address
> > > space on each add/remove callback, we wait until the commit phase
> > > and go through and rebuild a list by walking the Flatview of
> > > memory and end up producing an ordered list.
> > > We compare the list to the old list to trigger updates.
> > > 
> > > Note, only very lightly tested so far, I'm just trying to see if it's
> > > the right shape.
> > > 
> > > Igor, is this what you were intending?  
> > 
> > I was thinking about a little less intrusive approach
> > 
> > where vhost_region_add/del are modified to maintain
> > sorted by GPA array of mem_sections, vhost_dev::mem is dropped
> > altogether and vhost_memory_region array is build/used/freed
> > on every vhost_commit().
> > Maintaining sorted array should roughly cost us O(2 log n) if
> > binary search is used.
> > 
> > However I like your idea with iterator even more as it have
> > potential to make it even faster O(n) if we get rid of
> > quadratic and relatively complex vhost_update_compare_list().  
> 
> Note vhost_update_compare_list is complex,
> but it is O(n) - it's
> got nested loops, but the inner loop moves forward and oldi never
> gets reset back to zero.
While skimming through patches I've overlooked it.

Anyways,
why memcmp(old_arr, new_arr) is not sufficient
to detect a change in memory map?

> 
> > Pls, see comments on individual patches.  
> 
> Thanks; I have fixed a couple of bugs since I posted, so I'm
> more interested in structure/shape.  Any good ideas how to test
> it are welcome.
> 
> Dave
> 
> >   
> > > Dave
> > > 
> > > Dr. David Alan Gilbert (7):
> > >   memory: address_space_iterate
> > >   vhost: Move log_dirty check
> > >   vhost: New memory update functions
> > >   vhost: update_mem_cb implementation
> > >   vhost: Compare new and old memory lists
> > >   vhost: Copy updated region data into device state
> > >   vhost: Remove vhost_set_memory and children
> > > 
> > >  hw/virtio/trace-events |   8 +
> > >  hw/virtio/vhost.c      | 424 
> > > ++++++++++++++++++++++---------------------------
> > >  include/exec/memory.h  |  23 +++
> > >  memory.c               |  22 +++
> > >  4 files changed, 241 insertions(+), 236 deletions(-)
> > >   
> >   
> --
> Dr. David Alan Gilbert / address@hidden / Manchester, UK