[PATCH 0/15] KVM: optimize for MMIO handled

Xiao Guangrong
2011-06-07T12:56:09Z

The idea of this patchset is from Avi:
| We could cache the result of a miss in an spte by using a reserved bit, and
| checking the page fault error code (or seeing if we get an ept violation or
| ept misconfiguration), so if we get repeated mmio on a page, we don't need to
| search the slot list/tree.
| (https://lkml.org/lkml/2011/2/22/221)

The aim of this patchset is to support fast mmio emulate, it reduce searching
mmio gfn from memslots which is very expensive since we need to walk all slots
for mmio gfn, and the other advantage is: we can reduce guest page table walking
for soft mmu.

Lockless walk shadow page table is introduced in this patchset, it is the light
way to check the page fault is the real mmio page fault or something is running
out of our mind.

And, if shadow_notrap_nonpresent_pte is enabled(bypass_guest_pf=1), mmio page
fault and normal page fault is mixed(the reserved is set for all page fault),
it has little regression, if the box can generate lots of mmio access, for
example, the network server, it can disable shadow_notrap_nonpresent_pte and
enable mmio pf, after all, we can enable/disable mmio pf at the runtime.

The performance test result:

Netperf (TCP_RR):
===========================
ept is enabled:

Before After
1st 709.58 734.60
2nd 715.40 723.75
3rd 713.45 724.22

ept=0 bypass_guest_pf=0:

Before After
1st 706.10 709.63
2nd 709.38 715.80
3rd 695.90 710.70

Kernbech (do not redirect output to /dev/null)
==========================
ept is enabled:

Before After
1st 2m34.749s 2m33.482s
2nd 2m34.651s 2m33.161s
3rd 2m34.543s 2m34.271s

ept=0 bypass_guest_pf=0:

Before After
1st 4m43.467s 4m41.873s
2nd 4m45.225s 4m41.668s
3rd 4m47.029s 4m40.128s