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

[Dr. David Alan Gilbert]

* Igor Mammedov (address@hidden) wrote:
> On Thu, 30 Nov 2017 13:06:29 +0000
> "Dr. David Alan Gilbert" <address@hidden> wrote:
> 
> > * Igor Mammedov (address@hidden) wrote:
> > > On Thu, 30 Nov 2017 12:47:20 +0000
> > > "Dr. David Alan Gilbert" <address@hidden> wrote:
> > >   
> > > > * Igor Mammedov (address@hidden) wrote:  
> > > > > 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?    
> > > > 
> > > > It tells you that you've got a change, but doesn't give
> > > > the start/end of the range that's changed, and those
> > > > are used by vhost_commit to limit the work of
> > > > vhost_verify_ring_mappings.  
> > > Isn't memmap list a sorted and
> > >  dev->mem_changed_[start|end]_addr are the lowest|highest
> > > addresses of whole map?
> > > 
> > > If it's, so wouldn't getting values directly from 
> > > the 1st/last entries of array be sufficient?  
> > 
> > THat wasn't my understanding from the existing code;
> > my understanding was that changed_start_addr is set by
> > vhost_region_add->vhost_set_memory when a new region is added
> > (or removed) and is set to the limit of the section added.
> > But perhaps I'm misunderstanding.
> changed_*_addr is actually lower/upper bound of memory transaction
> and in practice it often includes several memory sections that
> get mapped during transaction (between begin - commit).
> 
> but then again,
>   - how expensive vhost_verify_ring_mappings() is?
>   - do we really need optimization here (perhaps finding out changed range is 
> more expensive)?
>   - how about calling vhost_verify_ring_mappings() unconditionally?

My worry is that:
    vhost_verify_ring_mappings
       vhost_verify_ring_part_mapping
          vhost_verify_ring_part_mapping
             vhost_memory_map & vhost_memory_unmap
               (non-iommu case...)
               cpu_physical_memory_map & cpu_physical_memory_unmap
                 address_space_map/address_space_unmap
                    flatview_translate etc

so it's not cheap at all - I *think* it should end up doing very little
after it's gone all the way through that because it should already be
mapped; but still it's not trivial.

Dave

> 
> > (The logic in vhost_verify_ring_mappings doesn't make sense
> > to me either though; if vhost_verify_ring_part_mapping returns 0
> > on success, why is it doing   if (!r) { break; }  surely it
> > should be  if (r) { break; })
> it looks like a bug (CCing Greg)
> 
> before (f1f9e6c5 vhost: adapt vhost_verify_ring_mappings() to virtio 1 ring 
> layout)
> logic used to be
> 
>    if changed_*_addr doesn't contain ring
>       "IGNORE as we don't care"
>    
>    if changed_*_addr contain ring AND ring can't be mapped at the same place
>       ABORT
> 
> with f1f9e6c5 we have 3 rings so on any of them following could happen
>    if "IGNORE as we don't care"
>      break => false success 
>      since it's possible that the remaining rings in vq do overlap and didn't 
> get checked
> 
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK