RE: [PATCH v3 12/13] x86/hyperv/vtl: Mark the wakeup mailbox page as private

[Michael Kelley]

From: Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx> Sent: Saturday, May 3, 2025 12:15 PM
> 
> From: Yunhong Jiang <yunhong.jiang@xxxxxxxxxxxxxxx>
> 
> The current code maps MMIO devices as shared (decrypted) by default in a
> confidential computing VM.
> 
> In a TDX environment, secondary CPUs are booted using the Multiprocessor
> Wakeup Structure defined in the ACPI specification. The virtual firmware
> and the operating system function in the guest context, without
> intervention from the VMM. Map the physical memory of the mailbox as
> private. Use the is_private_mmio() callback.
> 
> Signed-off-by: Yunhong Jiang <yunhong.jiang@xxxxxxxxxxxxxxx>
> Signed-off-by: Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx>
> ---
> Changes since v2:
>  - Use the new helper function get_mp_wakeup_mailbox_paddr().
>  - Edited the commit message for clarity.
> 
> Changes since v1:
>  - Added the helper function within_page() to improve readability
>  - Override the is_private_mmio() callback when detecting a TDX
>    environment. The address of the mailbox is checked in
>    hv_is_private_mmio_tdx().
> ---
>  arch/x86/hyperv/hv_vtl.c | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
> 
> diff --git a/arch/x86/hyperv/hv_vtl.c b/arch/x86/hyperv/hv_vtl.c
> index 8b497c8292d3..cd48bedd21f0 100644
> --- a/arch/x86/hyperv/hv_vtl.c
> +++ b/arch/x86/hyperv/hv_vtl.c
> @@ -54,6 +54,18 @@ static void  __noreturn hv_vtl_restart(char __maybe_unused *cmd)
>  	hv_vtl_emergency_restart();
>  }
> 
> +static inline bool within_page(u64 addr, u64 start)
> +{
> +	return addr >= start && addr < (start + PAGE_SIZE);
> +}
> +
> +static bool hv_vtl_is_private_mmio_tdx(u64 addr)
> +{
> +	u64 mb_addr = get_mp_wakeup_mailbox_paddr();
> +
> +	return mb_addr && within_page(addr, mb_addr);
> +}
> +
>  void __init hv_vtl_init_platform(void)
>  {
>  	pr_info("Linux runs in Hyper-V Virtual Trust Level\n");
> @@ -61,6 +73,8 @@ void __init hv_vtl_init_platform(void)
>  	/* There is no paravisor present if we are here. */
>  	if (hv_isolation_type_tdx()) {
>  		x86_init.resources.realmode_limit = SZ_4G;
> +		x86_platform.hyper.is_private_mmio = hv_vtl_is_private_mmio_tdx;
> +
>  	} else {
>  		x86_platform.realmode_reserve = x86_init_noop;
>  		x86_platform.realmode_init = x86_init_noop;
> --
> 2.43.0

Reviewed-by: Michael Kelley <mhklinux@xxxxxxxxxxx>