Re: [patch 00/13] RFC: out of sync shadow

Avi Kivity
2008-09-07T11:22:49Z

Marcelo Tosatti wrote:
> Keep shadow pages temporarily out of sync, allowing more efficient guest
> PTE updates in comparison to trap-emulate + unprotect heuristics. Stolen
> from Xen :)
>
> This version only allows leaf pagetables to go out of sync, for
> simplicity, but can be enhanced.
>
> VMX "bypass_guest_pf" feature on prefetch_page breaks it (since new
> PTE writes need no TLB flush, I assume). Not sure if its worthwhile to
> convert notrap_nonpresent -> trap_nonpresent on unshadow or just go
> for unconditional nonpaging_prefetch_page.
>
>

Doesn't it kill bypass_guest_pf completely? As soon as we unsync a
page, we can't have nontrapping nonpresent ptes in it.

We can try convertion on unsync, it does speed up demand paging.

> * Kernel builds on 4-way 64-bit guest improve 10% (+ 3.7% for
> get_user_pages_fast).
>
> * lmbench's "lat_proc fork" microbenchmark latency is 40% lower (a
> shadow worst scenario test).
>
> * The RHEL3 highpte kscand hangs go from 5+ seconds to < 1 second.
>
> * Windows 2003 Server, 32-bit PAE, DDK build (build -cPzM 3):
>
> Windows 2003 Checked 64 Bit Build Environment, 256M RAM
> 1-vcpu:
> vanilla + gup_fast: oos
> 0:04:37.375 0:03:28.047 (- 25%)
>
> 2-vcpus:
> vanilla + gup_fast oos
> 0:02:32.000 0:01:56.031 (- 23%)
>
>
> Windows 2003 Checked Build Environment, 1GB RAM
> 2-vcpus:
> vanilla + fast_gup oos
> 0:02:26.078 0:01:50.110 (- 24%)
>
> 4-vcpus:
> vanilla + fast_gup oos
> 0:01:59.266 0:01:29.625 (- 25%)
>
>

Impressive results.

> And I think other optimizations are possible now, for example the guest
> can be responsible for remote TLB flushing on kvm_mmu_pte_write().
>

But kvm_mmu_pte_write() is no longer called, since we unsync?

--
error compiling committee.c: too many arguments to function