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

[Michael Kelley]

From: Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxx>
> ---
> 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@xxxxxxxxxxx>