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


From: Dr. David Alan Gilbert
Subject: Re: [Qemu-devel] [RFC 0/7] Rework vhost memory region updates
Date: Thu, 30 Nov 2017 12:08:06 +0000
User-agent: Mutt/1.9.1 (2017-09-22)

* 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.

> 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



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