linux-kernel - [RFC 0/6] Enlightened VMCS support for KVM on Hyper-V

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Date:   Mon, 15 Jan 2018 18:30:59 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     kvm@...r.kernel.org
Cc:     x86@...nel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "Michael Kelley (EOSG)" <Michael.H.Kelley@...rosoft.com>,
        Mohammed Gamal <mmorsy@...hat.com>,
        Cathy Avery <cavery@...hat.com>, Bandan Das <bsd@...hat.com>,
        linux-kernel@...r.kernel.org
Subject: [RFC 0/6] Enlightened VMCS support for KVM on Hyper-V

Early RFC. I'll refer to this patchset in my DevConf/FOSDEM
presentations.

When running nested KVM on Hyper-V it's possible to use so called
'Enlightened VMCS' and do normal memory reads/writes instead of
doing VMWRITE/VMREAD instructions. Tests show that this speeds up
tight CPUID loop almost 3 times:

Before:
./cpuid_tight
20459

After:
./cpuid_tight
7698
    
checkpatch.pl errors/warnings and possible 32bit brokenness are known
things.
    
Main RFC questions I have are:
- Do we want to have this per L2 VM or per L1 host?
- How can we achieve zero overhead for non-Hyper-V deployments? Use static
  keys? But this will only work if we decide to do eVMCS per host.
- Can we do better than a big switch in evmcs_read()/evmcs_write()? And
  probably don't use 'case' defines which checkpatch.pl hates.

I would really apreciate any feedback!

Ladi Prosek (1):
  x86/kvm: rename HV_X64_MSR_APIC_ASSIST_PAGE to
    HV_X64_MSR_VP_ASSIST_PAGE

Vitaly Kuznetsov (5):
  x86/hyper-v: define virtual processor assist page structure
  x86/hyper-v: allocate and use hv_vp_assist_pages
  x86/hyper-v: define struct hv_enlightened_vmcs and clean field bits
  x86/hyper-v: detect nested features
  x86/kvm: use enlightened VMCS when running on Hyper-V

 arch/x86/hyperv/hv_init.c          |  35 ++-
 arch/x86/include/asm/mshyperv.h    |   3 +
 arch/x86/include/uapi/asm/hyperv.h | 223 +++++++++++++-
 arch/x86/kernel/cpu/mshyperv.c     |   3 +
 arch/x86/kvm/hyperv.c              |   8 +-
 arch/x86/kvm/lapic.h               |   2 +-
 arch/x86/kvm/vmx.c                 | 595 ++++++++++++++++++++++++++++++++++++-
 arch/x86/kvm/x86.c                 |   2 +-
 8 files changed, 857 insertions(+), 14 deletions(-)

-- 
2.14.3