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    Subject[PATCH v2 1/5] mshv: Add support for detecting nested hypervisor
    Date
    When Linux runs as a root partition for Microsoft Hypervisor. It is
    possible to detect if it is running as nested hypervisor using
    hints exposed by mshv. While at it expose a new variable called
    hv_nested which can be used later for making decisions specific to
    nested use case.

    Signed-off-by: Jinank Jain <jinankjain@linux.microsoft.com>
    ---
    arch/x86/include/asm/hyperv-tlfs.h | 3 +++
    arch/x86/include/asm/mshyperv.h | 2 ++
    arch/x86/kernel/cpu/mshyperv.c | 7 +++++++
    3 files changed, 12 insertions(+)

    diff --git a/arch/x86/include/asm/hyperv-tlfs.h b/arch/x86/include/asm/hyperv-tlfs.h
    index 3089ec352743..d9a611565859 100644
    --- a/arch/x86/include/asm/hyperv-tlfs.h
    +++ b/arch/x86/include/asm/hyperv-tlfs.h
    @@ -114,6 +114,9 @@
    /* Recommend using the newer ExProcessorMasks interface */
    #define HV_X64_EX_PROCESSOR_MASKS_RECOMMENDED BIT(11)

    +/* Indicates that the hypervisor is nested within a Hyper-V partition. */
    +#define HV_X64_HYPERV_NESTED BIT(12)
    +
    /* Recommend using enlightened VMCS */
    #define HV_X64_ENLIGHTENED_VMCS_RECOMMENDED BIT(14)

    diff --git a/arch/x86/include/asm/mshyperv.h b/arch/x86/include/asm/mshyperv.h
    index 61f0c206bff0..29388567eafd 100644
    --- a/arch/x86/include/asm/mshyperv.h
    +++ b/arch/x86/include/asm/mshyperv.h
    @@ -190,6 +190,8 @@ static inline void hv_ghcb_terminate(unsigned int set, unsigned int reason) {}

    extern bool hv_isolation_type_snp(void);

    +extern bool hv_nested;
    +
    static inline bool hv_is_synic_reg(unsigned int reg)
    {
    if ((reg >= HV_REGISTER_SCONTROL) &&
    diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
    index 831613959a92..2555535f5237 100644
    --- a/arch/x86/kernel/cpu/mshyperv.c
    +++ b/arch/x86/kernel/cpu/mshyperv.c
    @@ -37,6 +37,8 @@

    /* Is Linux running as the root partition? */
    bool hv_root_partition;
    +/* Is Linux running on nested Microsoft Hypervisor */
    +bool hv_nested;
    struct ms_hyperv_info ms_hyperv;

    #if IS_ENABLED(CONFIG_HYPERV)
    @@ -301,6 +303,11 @@ static void __init ms_hyperv_init_platform(void)
    pr_info("Hyper-V: running as root partition\n");
    }

    + if (ms_hyperv.hints & HV_X64_HYPERV_NESTED) {
    + hv_nested = true;
    + pr_info("Hyper-V: Linux running on a nested hypervisor\n");
    + }
    +
    /*
    * Extract host information.
    */
    --
    2.25.1
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