[Xen-devel] Re: [PATCH 0 of 4] mm+paravirt+xen: add pte read-modify-write abstraction

[Jeremy Fitzhardinge <jeremy@xxxxxxxx>]

Zachary Amsden wrote:
> I'm a bit skeptical you can get such a semantic to work without a very
> heavyweight method in the hypervisor.  How do you guarantee no other CPU
> is fizzling the A/D bits in the page table (it can be done by hardware
> with direct page tables), unless you use some kind of IPI?  Is this why
> it is still 7x?
>   
No, you just use cmpxchg.  It's pretty lightweight really.  Xen holds a 
lock internally to stop other cpus from updating the pte in software, so 
the only source of modification is the hardware itself; the cmpxchg loop 
is guaranteed to terminate because the A/D bits can only transition from 
0->1.
I haven't really gone into depth as to exactly where the 7x number comes 
from.  I could increase the batch size (currently max of 32 pte 
updates/hypercall), and some of it is plain overhead from the in-kernel 
infrastructure.  A simpler and more hackish approach which basically 
pastes the Xen hypercall directly into the mprotect loop gets the 
overhead down to about 5.5x.
> Still, a 7x gain from asynchronous batching is very nice.  I wonder if
> that means the average mprotect size in your benchmark is 7 pages.
>   
Yeah, it's around 7x.  The batching pays off even for single page 
mprotects, because the trap and emulate of xchg is so expensive.

> > I believe that other virtualization systems, whether they use direct
> > paging like Xen, or a shadow pagetable scheme (vmi, kvm, lguest), can
> > make use of this interface to improve the performance.
> >     
> On VMI, we don't trap the xchg of the pte, thus we don't have any
> bottleneck here to begin with.
If you're doing code rewriting then I guess you can effectively do the 
same trick at that point.  If not, then presumably you take a fault for 
the first pte updated in the mprotect and then sync the shadow up when 
the tlb flush happens; batching that trap and the tlb flush would give 
you some benefit for small mprotects.
   J

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