linux-kernel - RE: [PATCH v3 13/13] x86/hyperv/vtl: Use the wakeup mailbox to boot secondary CPUs

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Message-ID:
 <SN6PR02MB4157F93812247213DFA922ECD49FA@SN6PR02MB4157.namprd02.prod.outlook.com>
Date: Tue, 20 May 2025 01:35:02 +0000
From: Michael Kelley <mhklinux@...look.com>
To: Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>, "x86@...nel.org"
	<x86@...nel.org>, Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley
	<conor+dt@...nel.org>, Rob Herring <robh@...nel.org>, "K. Y. Srinivasan"
	<kys@...rosoft.com>, Haiyang Zhang <haiyangz@...rosoft.com>, Wei Liu
	<wei.liu@...nel.org>, Dexuan Cui <decui@...rosoft.com>
CC: "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>, Saurabh Sengar
	<ssengar@...ux.microsoft.com>, Chris Oo <cho@...rosoft.com>,
	"linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>, "Kirill A.
 Shutemov" <kirill.shutemov@...ux.intel.com>, "linux-acpi@...r.kernel.org"
	<linux-acpi@...r.kernel.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
	Ricardo Neri <ricardo.neri@...el.com>
Subject: RE: [PATCH v3 13/13] x86/hyperv/vtl: Use the wakeup mailbox to boot
 secondary CPUs

From: Ricardo Neri <ricardo.neri-calderon@...ux.intel.com> Sent: Saturday, May 3, 2025 12:15 PM
> 
> The hypervisor is an untrusted entity for TDX guests. It cannot be used
> to boot secondary CPUs. The function hv_vtl_wakeup_secondary_cpu() cannot
> be used.
> 
> Instead, the virtual firmware boots the secondary CPUs and places them in
> a state to transfer control to the kernel using the wakeup mailbox.
> 
> The kernel updates the APIC callback wakeup_secondary_cpu_64() to use
> the mailbox if detected early during boot (enumerated via either an ACPI
> table or a DeviceTree node).
> 
> Signed-off-by: Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
> ---
> Changes since v2:
>  - Unconditionally use the wakeup mailbox in a TDX confidential VM.
>    (Michael).
>  - Edited the commit message for clarity.
> 
> Changes since v1:
>  - None
> ---
>  arch/x86/hyperv/hv_vtl.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/hyperv/hv_vtl.c b/arch/x86/hyperv/hv_vtl.c
> index cd48bedd21f0..30a5a0c156c1 100644
> --- a/arch/x86/hyperv/hv_vtl.c
> +++ b/arch/x86/hyperv/hv_vtl.c
> @@ -299,7 +299,15 @@ int __init hv_vtl_early_init(void)
>  		panic("XSAVE has to be disabled as it is not supported by this module.\n"
>  			  "Please add 'noxsave' to the kernel command line.\n");
> 
> -	apic_update_callback(wakeup_secondary_cpu_64, hv_vtl_wakeup_secondary_cpu);
> +	/*
> +	 * TDX confidential VMs do not trust the hypervisor and cannot use it to
> +	 * boot secondary CPUs. Instead, they will be booted using the wakeup
> +	 * mailbox if detected during boot. See setup_arch().
> +	 *
> +	 * There is no paravisor present if we are here.
> +	 */
> +	if (!hv_isolation_type_tdx())
> +		apic_update_callback(wakeup_secondary_cpu_64, hv_vtl_wakeup_secondary_cpu);
> 
>  	return 0;
>  }
> --
> 2.43.0

Reviewed-by: Michael Kelley <mhklinux@...look.com>