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

Marcelo Tosatti
2008-09-06T19:26:50Z

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.

* 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%)

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

Please review.

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