Re: [RFC PATCH v2 00/11] Intel IA32_SPEC_CTRL Virtualization

[Binbin Wu]

On 4/14/2023 2:25 PM, Chao Gao wrote:
> Changes since RFC v1:
>   * add two kselftests (patch 10-11)
>   * set virtual MSRs also on APs [Pawan]
>   * enable "virtualize IA32_SPEC_CTRL" for L2 to prevent L2 from changing
>     some bits of IA32_SPEC_CTRL (patch 4)
>   * other misc cleanup and cosmetic changes
>
> RFC v1: https://lore.kernel.org/lkml/20221210160046.2608762-1-chen.zhang@xxxxxxxxx/
>
>
> This series introduces "virtualize IA32_SPEC_CTRL" support. Here are
> introduction and use cases of this new feature.
>
> ### Virtualize IA32_SPEC_CTRL
>
> "Virtualize IA32_SPEC_CTRL" [1] is a new VMX feature on Intel CPUs. This feature
> allows VMM to lock some bits of IA32_SPEC_CTRL MSR even when the MSR is
> pass-thru'd to a guest.
>
>
> ### Use cases of "virtualize IA32_SPEC_CTRL" [2]
>
> Software mitigations like Retpoline and software BHB-clearing sequence depend on
> CPU microarchitectures. And guest cannot know exactly the underlying
> microarchitecture. When a guest is migrated between processors of different
> microarchitectures, software mitigations which work perfectly on previous
> microachitecture may be not effective on the new one. To fix the problem, some
> hardware mitigations should be used in conjunction with software mitigations.

So even the hardware mitigations are enabled, the software mitigations 
are still needed, right?


> Using virtual IA32_SPEC_CTRL, VMM can enforce hardware mitigations transparently
> to guests and avoid those hardware mitigations being unintentionally disabled
> when guest changes IA32_SPEC_CTRL MSR.
>
>
> ### Intention of this series
>
> This series adds the capability of enforcing hardware mitigations for guests
> transparently and efficiently (i.e., without intecepting IA32_SPEC_CTRL MSR

/s/intecepting/intercepting


> accesses) to kvm. The capability can be used to solve the VM migration issue in
> a pool consisting of processors of different microarchitectures.
>
> Specifically, below are two target scenarios of this series:
>
> Scenario 1: If retpoline is used by a VM to mitigate IMBTI in CPL0, VMM can set
> 	    RRSBA_DIS_S on parts enumerates RRSBA. Note that the VM is presented
> 	    with a microarchitecture doesn't enumerate RRSBA.
>
> Scenario 2: If a VM uses software BHB-clearing sequence on transitions into CPL0
> 	    to mitigate BHI, VMM can use "virtualize IA32_SPEC_CTRL" to set
> 	    BHI_DIS_S on new parts which doesn't enumerate BHI_NO.
>
> Intel defines some virtual MSRs [2] for guests to report in-use software
> mitigations. This allows guests to opt in VMM's deploying hardware mitigations
> for them if the guests are either running or later migrated to a system on which
> in-use software mitigations are not effective. The virtual MSRs interface is
> also added in this series.
>
> ### Organization of this series
>
> 1. Patch 1-3	Advertise RRSBA_CTRL and BHI_CTRL to guest
> 2. Patch 4	Add "virtualize IA32_SPEC_CTRL" support
> 3. Patch 5-9	Allow guests to report in-use software mitigations to KVM so
>                  that KVM can enable hardware mitigations for guests.
> 4. Patch 10-11	Add kselftest for virtual MSRs and IA32_SPEC_CTRL
>
> [1]: https://cdrdv2.intel.com/v1/dl/getContent/671368 Ref. #319433-047 Chapter 12
> [2]: https://www.intel.com/content/www/us/en/developer/articles/technical/software-security-guidance/technical-documentation/branch-history-injection.html
>
> Chao Gao (3):
>    KVM: VMX: Advertise MITI_ENUM_RETPOLINE_S_SUPPORT
>    KVM: selftests: Add tests for virtual enumeration/mitigation MSRs
>    KVM: selftests: Add tests for IA32_SPEC_CTRL MSR
>
> Pawan Gupta (1):
>    x86/bugs: Use Virtual MSRs to request hardware mitigations
>
> Zhang Chen (7):
>    x86/msr-index: Add bit definitions for BHI_DIS_S and BHI_NO
>    KVM: x86: Advertise CPUID.7.2.EDX and RRSBA_CTRL support
>    KVM: x86: Advertise BHI_CTRL support
>    KVM: VMX: Add IA32_SPEC_CTRL virtualization support
>    KVM: x86: Advertise ARCH_CAP_VIRTUAL_ENUM support
>    KVM: VMX: Advertise MITIGATION_CTRL support
>    KVM: VMX: Advertise MITI_CTRL_BHB_CLEAR_SEQ_S_SUPPORT
>
>   arch/x86/include/asm/msr-index.h              |  33 +++-
>   arch/x86/include/asm/vmx.h                    |   5 +
>   arch/x86/include/asm/vmxfeatures.h            |   2 +
>   arch/x86/kernel/cpu/bugs.c                    |  25 +++
>   arch/x86/kvm/cpuid.c                          |  22 ++-
>   arch/x86/kvm/reverse_cpuid.h                  |   8 +
>   arch/x86/kvm/svm/svm.c                        |   3 +
>   arch/x86/kvm/vmx/capabilities.h               |   5 +
>   arch/x86/kvm/vmx/nested.c                     |  13 ++
>   arch/x86/kvm/vmx/vmcs.h                       |   2 +
>   arch/x86/kvm/vmx/vmx.c                        | 112 ++++++++++-
>   arch/x86/kvm/vmx/vmx.h                        |  43 ++++-
>   arch/x86/kvm/x86.c                            |  19 +-
>   tools/arch/x86/include/asm/msr-index.h        |  37 +++-
>   tools/testing/selftests/kvm/Makefile          |   2 +
>   .../selftests/kvm/include/x86_64/processor.h  |   5 +
>   .../selftests/kvm/x86_64/spec_ctrl_msr_test.c | 178 ++++++++++++++++++
>   .../kvm/x86_64/virtual_mitigation_msr_test.c  | 175 +++++++++++++++++
>   18 files changed, 676 insertions(+), 13 deletions(-)
>   create mode 100644 tools/testing/selftests/kvm/x86_64/spec_ctrl_msr_test.c
>   create mode 100644 tools/testing/selftests/kvm/x86_64/virtual_mitigation_msr_test.c
>
>
> base-commit: 400d2132288edbd6d500f45eab5d85526ca94e46