RE: [PATCH v3 10/13] x86/hyperv/vtl: Setup the 64-bit trampoline for TDX guests

[Michael Kelley]

From: Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx> Sent: Saturday, May 3, 2025 12:15 PM

> 
> From: Yunhong Jiang <yunhong.jiang@xxxxxxxxxxxxxxx>
> 
> The hypervisor is an untrusted entity for TDX guests. It cannot be used
> to boot secondary CPUs - neither via hypercalls not the INIT assert,
> de-assert plus Start-Up IPI messages.
> 
> Instead, the platform virtual firmware boots the secondary CPUs and
> puts them in a state to transfer control to the kernel. This mechanism uses
> the wakeup mailbox described in the Multiprocessor Wakeup Structure of the
> ACPI specification. The entry point to the kernel is trampoline_start64.
> 
> Allocate and setup the trampoline using the default x86_platform callbacks.
> 
> The platform firmware configures the secondary CPUs in long mode. It is no
> longer necessary to locate the trampoline under 1MB memory. After handoff
> from firmware, the trampoline code switches briefly to 32-bit addressing
> mode, which has an addressing limit of 4GB. Set the upper bound of the
> trampoline memory accordingly.
> 
> Signed-off-by: Yunhong Jiang <yunhong.jiang@xxxxxxxxxxxxxxx>
> Signed-off-by: Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx>
> ---
> Changes since v2:
>  - Added a note regarding there is no need to check for a present
>    paravisor.
>  - Edited commit message for clarity.
> 
> Changes since v1:
>  - Dropped the function hv_reserve_real_mode(). Instead, used the new
>    members realmode_limit and reserve_bios members of x86_init to
>    set the upper bound of the trampoline memory. (Thomas)
> ---
>  arch/x86/hyperv/hv_vtl.c | 11 ++++++++---
>  1 file changed, 8 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/hyperv/hv_vtl.c b/arch/x86/hyperv/hv_vtl.c
> index 6bd183ee484f..8b497c8292d3 100644
> --- a/arch/x86/hyperv/hv_vtl.c
> +++ b/arch/x86/hyperv/hv_vtl.c
> @@ -58,9 +58,14 @@ void __init hv_vtl_init_platform(void)
>  {
>  	pr_info("Linux runs in Hyper-V Virtual Trust Level\n");
> 
> -	x86_platform.realmode_reserve = x86_init_noop;
> -	x86_platform.realmode_init = x86_init_noop;
> -	real_mode_header = &hv_vtl_real_mode_header;
> +	/* There is no paravisor present if we are here. */
> +	if (hv_isolation_type_tdx()) {
> +		x86_init.resources.realmode_limit = SZ_4G;
> +	} else {
> +		x86_platform.realmode_reserve = x86_init_noop;
> +		x86_platform.realmode_init = x86_init_noop;
> +		real_mode_header = &hv_vtl_real_mode_header;
> +	}
>  	x86_init.irqs.pre_vector_init = x86_init_noop;
>  	x86_init.timers.timer_init = x86_init_noop;
>  	x86_init.resources.probe_roms = x86_init_noop;
> --
> 2.43.0

Reviewed-by: Michael Kelley <mhklinux@xxxxxxxxxxx>