linux-kernel - [PATCH 0/5] KVM: x86: Partially allow KVM_SET_CPUID{,2} after KVM

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Date:   Thu, 13 Jan 2022 14:36:58 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Igor Mammedov <imammedo@...hat.com>,
        linux-kernel@...r.kernel.org
Subject: [PATCH 0/5] KVM: x86: Partially allow KVM_SET_CPUID{,2} after KVM_RUN for CPU hotplug

Recently, KVM made it illegal to change CPUID after KVM_RUN but
unfortunately this change is not fully compatible with existing VMMs.
In particular, QEMU reuses vCPU fds for CPU hotplug after unplug and it
calls KVM_SET_CPUID2. Relax the requirement by implementing an allowlist
of entries which are allowed to change.

Going forward, VMMs are supposed to change the behavior. There is no real
need to change CPUID information. For hotplug purposes (and if reusing vCPU
fds is still considered being worthy), VMMs can be a bit smarter and always
pick the fd with the required LAPIC/x2APIC id to eliminate the need to
change this info later.

Vitaly Kuznetsov (5):
  KVM: x86: Fix indentation in kvm_set_cpuid()
  KVM: x86: Do runtime CPUID update before updating
    vcpu->arch.cpuid_entries
  KVM: x86: Partially allow KVM_SET_CPUID{,2} after KVM_RUN
  KVM: selftests: Rename 'get_cpuid_test' to 'cpuid_test'
  KVM: selftests: Test KVM_SET_CPUID2 after KVM_RUN

 arch/x86/kvm/cpuid.c                          | 120 ++++++++++++++----
 arch/x86/kvm/x86.c                            |  19 ---
 tools/testing/selftests/kvm/.gitignore        |   2 +-
 tools/testing/selftests/kvm/Makefile          |   5 +-
 .../selftests/kvm/include/x86_64/processor.h  |   7 +
 .../selftests/kvm/lib/x86_64/processor.c      |  33 ++++-
 .../x86_64/{get_cpuid_test.c => cpuid_test.c} |  78 ++++++++++++
 7 files changed, 216 insertions(+), 48 deletions(-)
 rename tools/testing/selftests/kvm/x86_64/{get_cpuid_test.c => cpuid_test.c} (66%)

-- 
2.34.1