[PATCH 0/3] KVM: x86: Support hibernation of a nested Hyper-V

[Mushahid Hussain <hmushi@amazon.co.uk>]

A Windows guest with the Hyper-V role enabled runs its own hypervisor
(hvix64) nested under KVM. When KVM advertises the Hyper-V time and
frequency MSRs, hvix64 lets KVM own the partition reference time. It
then offers hibernation to Windows only if KVM also advertises CPUID
0x40000004 EAX bit 20 (RestoreTimeOnResume), and on resume restores
the saved TSC and reference time through HvCallRestorePartitionTime
(0x103). Both are reserved in the TLFS and defined in Microsoft's
hvdef crate.

Patch 1 fixes a stale nVMX sync flag that zeroed the vmcs12 segment
state hvix64 reloads on resume. It stands alone and is tagged for
stable. Patch 2 is a refactor with no functional change. Patch 3
advertises the bit and implements the hypercall, restoring kvmclock
and every vCPU's TSC inside one pvclock update.

Tested on Windows Server 2025 with Hyper-V running, guest initiated
hibernate and resume, under QEMU with a matching CPU property:
https://github.com/hmushi/qemu/tree/hv-restore-time-on-resume

Mushahid Hussain (3):
  KVM: nVMX: Clear stale vmcs02 sync flag in free_nested()
  KVM: x86: Extract __kvm_set_clock() from kvm_vm_ioctl_set_clock()
  KVM: x86: hyper-v: Implement HvCallRestorePartitionTime

 arch/x86/include/asm/kvm_host.h |   5 ++
 arch/x86/kvm/hyperv.c           |  36 +++++++++
 arch/x86/kvm/vmx/nested.c       |   1 +
 arch/x86/kvm/x86.c              | 131 +++++++++++++++++++++++++++-----
 arch/x86/kvm/x86.h              |   1 +
 include/hyperv/hvgdk_mini.h     |  12 +++
 6 files changed, 168 insertions(+), 18 deletions(-)


base-commit: 6bd2905303c58679e303941b7d8ae8c074cd95ce
-- 
2.47.3