linux-kernel - RE: [PATCH v2 1/5] mshv: Add support for detecting nested hypervisor

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Date:   Thu, 3 Nov 2022 03:29:32 +0000
From:   "Michael Kelley (LINUX)" <mikelley@...rosoft.com>
To:     Jinank Jain <jinankjain@...ux.microsoft.com>,
        Jinank Jain <jinankjain@...rosoft.com>
CC:     KY Srinivasan <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        "sthemmin@...rosoft.com" <sthemmin@...rosoft.com>,
        "wei.liu@...nel.org" <wei.liu@...nel.org>,
        Dexuan Cui <decui@...rosoft.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
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        "sathyanarayanan.kuppuswamy@...ux.intel.com" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
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Subject: RE: [PATCH v2 1/5] mshv: Add support for detecting nested hypervisor

From: Jinank Jain <jinankjain@...ux.microsoft.com> Sent: Wednesday, November 2, 2022 9:36 AM
> 

The email subject prefix for a patch like this is usually "x86/hyperv"
Check the commit log for arch/x86/kernel/cpu/mshyperv.c, and you'll see
what is usually done.  It's best to maintain consistency.

Same comment applies to other patches in this series:  Check the
commit log for the relevant files and pick the prefix that fits best.

> 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@...ux.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;

This reference to hv_nested will cause problems similar to what happens
with hv_root_partition.   See the comments in hv_common.c about how
hv_root_partition is handled.  I think you'll need to do the same thing
with hv_nested.

> +		pr_info("Hyper-V: Linux running on a nested hypervisor\n");

A nit:  Let's drop the word "Linux" from the above message so it is
consistent with the previous message about "running as root partition".

> +	}
> +
>  	/*
>  	 * Extract host information.
>  	 */
> --
> 2.25.1