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Subject[PATCH 0/5] KVM: x86: hyperv: PV TLB flush for Windows guests
Date
This is both a new feature and a bugfix.

Bugfix description:

It was found that Windows 2016 guests on KVM crash when they have > 64
vCPUs, non-flat topology (>1 core/thread per socket; in case it has >64
sockets Windows just ignores vCPUs above 64) and Hyper-V enlightenments
(any) are enabled. The most common error reported is "PAGE FAULT IN
NONPAGED AREA" but I saw different messages. Apparently, Windows doesn't
expect to run on a Hyper-V server without PV TLB flush support as there's
no such Hyper-V servers out there (it's only WS2016 supporting > 64 vCPUs
AFAIR).

Adding PV TLB flush support to KVM helps, Windows 2016 guests now boot
normally (I tried '-smp 128,sockets=64,cores=1,threads=2' and
'-smp 128,sockets=8,cores=16,threads=1' but other topologies should work
too).

Feature description:

PV TLB flush helps a lot when running overcommited. KVM gained support for
it recently but it is only available for Linux guests. Windows guests use
emulated Hyper-V interface and PV TLB flush needs to be added there.

I tested WS2016 guest with 128 vCPUs running on a 12 pCPU server. The test
was running 64 threads doing 100 mmap()/munmap() for 16384 pages with a
tiny random nanosleep in between (I used Cygwin. It would be great if
someone could point me to a good Windows-native TLB trashing test).

The results are:
Before:
real 0m44.362s
user 0m1.796s
sys 6m43.218s

After:
real 0m24.425s
user 0m1.811s
sys 0m40.625s

When running without overcommit (single 12 vCPU guest on 12 pCPU server) the
results of the same test are very close:
Before:
real 0m21.237s
user 0m1.531s
sys 0m19.984s

After:
real 0m21.082s
user 0m1.546s
sys 0m20.030s

Implementation details.

The implementation is very simplistic and straightforward. We ignore
'address space' argument of the hypercalls (as there is no good way to
figure out what's currently in CR3 of a running vCPU as generally we don't
VMEXIT on guest CR3 write) and do full TLB flush on specified vCPUs. In
case said vCPUs are not running TLB flush will be performed upon guest
enter.

Qemu (and other userspaces) need to enable CPUID feature bits to make
Windows aware the feature is supported. I'll post Qemu enablement patch
separately.

Patches are based on the current kvm/queue branch.

Vitaly Kuznetsov (5):
x86/hyper-v: move struct hv_flush_pcpu{,ex} definitions to common
header
KVM: x86: hyperv: use defines when parsing hypercall parameters
KVM: x86: hyperv: simplistic HVCALL_FLUSH_VIRTUAL_ADDRESS_{LIST,SPACE}
implementation
KVM: x86: hyperv: simplistic
HVCALL_FLUSH_VIRTUAL_ADDRESS_{LIST,SPACE}_EX implementation
KVM: x86: hyperv: declare KVM_CAP_HYPERV_TLBFLUSH capability

Documentation/virtual/kvm/api.txt | 9 +++
arch/x86/hyperv/mmu.c | 40 +++-------
arch/x86/include/asm/hyperv-tlfs.h | 20 +++++
arch/x86/kvm/hyperv.c | 154 ++++++++++++++++++++++++++++++++++---
arch/x86/kvm/trace.h | 51 ++++++++++++
arch/x86/kvm/x86.c | 1 +
include/uapi/linux/kvm.h | 1 +
7 files changed, 236 insertions(+), 40 deletions(-)

--
2.14.3

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