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From:   Michael Kelley <mikelley@microsoft.com>
To:     Wei Liu <wei.liu@kernel.org>,
        Linux on Hyper-V List <linux-hyperv@vger.kernel.org>
CC:     "virtualization@lists.linux-foundation.org" 
        <virtualization@lists.linux-foundation.org>,
        Linux Kernel List <linux-kernel@vger.kernel.org>,
        Vineeth Pillai <viremana@linux.microsoft.com>,
        Sunil Muthuswamy <sunilmut@microsoft.com>,
        Nuno Das Neves <nunodasneves@linux.microsoft.com>,
        "pasha.tatashin@soleen.com" <pasha.tatashin@soleen.com>,
        KY Srinivasan <kys@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>
Subject: RE: [PATCH v5 02/16] x86/hyperv: detect if Linux is the root
 partition
Thread-Topic: [PATCH v5 02/16] x86/hyperv: detect if Linux is the root
 partition
Thread-Index: AQHW7yPvqzTQq44n9EKlBqAzieFq1qo5FUJA
Date:   Tue, 26 Jan 2021 00:31:31 +0000
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 <20210120120058.29138-3-wei.liu@kernel.org>
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From: Wei Liu <wei.liu@kernel.org> Sent: Wednesday, January 20, 2021 4:01 A=
M
>=20
> For now we can use the privilege flag to check. Stash the value to be
> used later.
>=20
> Put in a bunch of defines for future use when we want to have more
> fine-grained detection.
>=20
> Signed-off-by: Wei Liu <wei.liu@kernel.org>
> ---
> v3: move hv_root_partition to mshyperv.c
> ---
>  arch/x86/include/asm/hyperv-tlfs.h | 10 ++++++++++
>  arch/x86/include/asm/mshyperv.h    |  2 ++
>  arch/x86/kernel/cpu/mshyperv.c     | 20 ++++++++++++++++++++
>  3 files changed, 32 insertions(+)
>=20
> diff --git a/arch/x86/include/asm/hyperv-tlfs.h b/arch/x86/include/asm/hy=
perv-tlfs.h
> index 6bf42aed387e..204010350604 100644
> --- a/arch/x86/include/asm/hyperv-tlfs.h
> +++ b/arch/x86/include/asm/hyperv-tlfs.h
> @@ -21,6 +21,7 @@
>  #define HYPERV_CPUID_FEATURES			0x40000003
>  #define HYPERV_CPUID_ENLIGHTMENT_INFO		0x40000004
>  #define HYPERV_CPUID_IMPLEMENT_LIMITS		0x40000005
> +#define HYPERV_CPUID_CPU_MANAGEMENT_FEATURES	0x40000007
>  #define HYPERV_CPUID_NESTED_FEATURES		0x4000000A
>=20
>  #define HYPERV_CPUID_VIRT_STACK_INTERFACE	0x40000081
> @@ -110,6 +111,15 @@
>  /* Recommend using enlightened VMCS */
>  #define HV_X64_ENLIGHTENED_VMCS_RECOMMENDED		BIT(14)
>=20
> +/*
> + * CPU management features identification.
> + * These are HYPERV_CPUID_CPU_MANAGEMENT_FEATURES.EAX bits.
> + */
> +#define HV_X64_START_LOGICAL_PROCESSOR			BIT(0)
> +#define HV_X64_CREATE_ROOT_VIRTUAL_PROCESSOR		BIT(1)
> +#define HV_X64_PERFORMANCE_COUNTER_SYNC			BIT(2)
> +#define HV_X64_RESERVED_IDENTITY_BIT			BIT(31)
> +

I wonder if these bit definitions should go in the asm-generic part of
hyperv-tlfs.h instead of the X64 specific part.  They look very architectur=
e
neutral (in which case the X64 should be dropped from the name
as well).  Of course, they can be moved later when/if we get to that point
and have a firmer understanding of what is and isn't arch neutral.

>  /*
>   * Virtual processor will never share a physical core with another virtu=
al
>   * processor, except for virtual processors that are reported as sibling=
 SMT
> diff --git a/arch/x86/include/asm/mshyperv.h b/arch/x86/include/asm/mshyp=
erv.h
> index ffc289992d1b..ac2b0d110f03 100644
> --- a/arch/x86/include/asm/mshyperv.h
> +++ b/arch/x86/include/asm/mshyperv.h
> @@ -237,6 +237,8 @@ int hyperv_fill_flush_guest_mapping_list(
>  		struct hv_guest_mapping_flush_list *flush,
>  		u64 start_gfn, u64 end_gfn);
>=20
> +extern bool hv_root_partition;
> +
>  #ifdef CONFIG_X86_64
>  void hv_apic_init(void);
>  void __init hv_init_spinlocks(void);
> diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyper=
v.c
> index f628e3dc150f..c376d191a260 100644
> --- a/arch/x86/kernel/cpu/mshyperv.c
> +++ b/arch/x86/kernel/cpu/mshyperv.c
> @@ -32,6 +32,10 @@
>  #include <asm/nmi.h>
>  #include <clocksource/hyperv_timer.h>
>=20
> +/* Is Linux running as the root partition? */
> +bool hv_root_partition;
> +EXPORT_SYMBOL_GPL(hv_root_partition);
> +
>  struct ms_hyperv_info ms_hyperv;
>  EXPORT_SYMBOL_GPL(ms_hyperv);
>=20
> @@ -237,6 +241,22 @@ static void __init ms_hyperv_init_platform(void)
>  	pr_debug("Hyper-V: max %u virtual processors, %u logical processors\n",
>  		 ms_hyperv.max_vp_index, ms_hyperv.max_lp_index);
>=20
> +	/*
> +	 * Check CPU management privilege.
> +	 *
> +	 * To mirror what Windows does we should extract CPU management
> +	 * features and use the ReservedIdentityBit to detect if Linux is the
> +	 * root partition. But that requires negotiating CPU management
> +	 * interface (a process to be finalized).
> +	 *
> +	 * For now, use the privilege flag as the indicator for running as
> +	 * root.
> +	 */
> +	if (cpuid_ebx(HYPERV_CPUID_FEATURES) & HV_CPU_MANAGEMENT) {

Should the EBX value be captured in the ms_hyperv structure with the
other similar values, and then used from there?

Michael

> +		hv_root_partition =3D true;
> +		pr_info("Hyper-V: running as root partition\n");
> +	}
> +
>  	/*
>  	 * Extract host information.
>  	 */
> --
> 2.20.1