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Subject: Re: [PATCH v8 1/5] x86/hyperv: Add support for detecting nested
 hypervisor
To:     Jinank Jain <jinankjain@linux.microsoft.com>,
        jinankjain@microsoft.com
Cc:     kys@microsoft.com, haiyangz@microsoft.com, wei.liu@kernel.org,
        decui@microsoft.com, tglx@linutronix.de, mingo@redhat.com,
        bp@alien8.de, dave.hansen@linux.intel.com, x86@kernel.org,
        hpa@zytor.com, arnd@arndb.de, peterz@infradead.org,
        jpoimboe@kernel.org, seanjc@google.com,
        kirill.shutemov@linux.intel.com, ak@linux.intel.com,
        sathyanarayanan.kuppuswamy@linux.intel.com,
        linux-hyperv@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-arch@vger.kernel.org, anrayabh@linux.microsoft.com,
        mikelley@microsoft.com
References: <cover.1670561320.git.jinankjain@linux.microsoft.com>
 <8e3e7112806e81d2292a66a56fe547162754ecea.1670561320.git.jinankjain@linux.microsoft.com>
From:   Nuno Das Neves <nunodasneves@linux.microsoft.com>
Message-ID: <c6bc34aa-c986-b68c-a19e-19670b2f72b7@linux.microsoft.com>
Date:   Fri, 9 Dec 2022 09:58:55 -0800
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On 12/8/2022 9:32 PM, Jinank Jain wrote:
> Detect if Linux is running as a nested hypervisor in the root
> partition for Microsoft Hypervisor, using flags provided by MSHV.
> Expose a new variable hv_nested that is used later for decisions
> specific to the nested use case.
> 
> Signed-off-by: Jinank Jain <jinankjain@linux.microsoft.com>
> ---
>  arch/x86/include/asm/hyperv-tlfs.h | 3 +++
>  arch/x86/kernel/cpu/mshyperv.c     | 7 +++++++
>  drivers/hv/hv_common.c             | 9 ++++++---
>  include/asm-generic/mshyperv.h     | 1 +
>  4 files changed, 17 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/include/asm/hyperv-tlfs.h b/arch/x86/include/asm/hyperv-tlfs.h
> index 6d9368ea3701..58c03d18c235 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/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
> index 46668e255421..f9b78d4829e3 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: running on a nested hypervisor\n");
> +	}
> +
>  	/*
>  	 * Extract host information.
>  	 */
> diff --git a/drivers/hv/hv_common.c b/drivers/hv/hv_common.c
> index ae68298c0dca..52a6f89ccdbd 100644
> --- a/drivers/hv/hv_common.c
> +++ b/drivers/hv/hv_common.c
> @@ -25,17 +25,20 @@
>  #include <asm/mshyperv.h>
>  
>  /*
> - * hv_root_partition and ms_hyperv are defined here with other Hyper-V
> - * specific globals so they are shared across all architectures and are
> + * hv_root_partition, ms_hyperv and hv_nested are defined here with other
> + * Hyper-V specific globals so they are shared across all architectures and are
>   * built only when CONFIG_HYPERV is defined.  But on x86,
>   * ms_hyperv_init_platform() is built even when CONFIG_HYPERV is not
> - * defined, and it uses these two variables.  So mark them as __weak
> + * defined, and it uses these three variables.  So mark them as __weak
>   * here, allowing for an overriding definition in the module containing
>   * ms_hyperv_init_platform().
>   */
>  bool __weak hv_root_partition;
>  EXPORT_SYMBOL_GPL(hv_root_partition);
>  
> +bool __weak hv_nested;
> +EXPORT_SYMBOL_GPL(hv_nested);
> +
>  struct ms_hyperv_info __weak ms_hyperv;
>  EXPORT_SYMBOL_GPL(ms_hyperv);
>  
> diff --git a/include/asm-generic/mshyperv.h b/include/asm-generic/mshyperv.h
> index bfb9eb9d7215..f131027830c3 100644
> --- a/include/asm-generic/mshyperv.h
> +++ b/include/asm-generic/mshyperv.h
> @@ -48,6 +48,7 @@ struct ms_hyperv_info {
>  	u64 shared_gpa_boundary;
>  };
>  extern struct ms_hyperv_info ms_hyperv;
> +extern bool hv_nested;
>  
>  extern void * __percpu *hyperv_pcpu_input_arg;
>  extern void * __percpu *hyperv_pcpu_output_arg;
> 

Reviewed-by: Nuno Das Neves <nunodasneves@linux.microsoft.com>