Re: [PATCH v8 09/10] x86/hyperv/vtl: Mark the wakeup mailbox page as private

[Wei Liu]

Dexuan, are you happy with the patch? You can also delegate to Saurabh
if you think it's more appropriate. Thanks!

On Wed, Jan 07, 2026 at 01:44:45PM -0800, Ricardo Neri wrote:
> 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 in v8:
>  - Included linux/acpi.h to add missing definitions that caused build
>    breaks (kernel test robot)
> 
> Changes in v7:
>  - Dropped check for !CONFIG_X86_MAILBOX_WAKEUP. The symbol is no longer
>    valid and now we have a stub for !CONFIG_ACPI.
>  - Dropped Reviewed-by tags from Dexuan and Michael as this patch
>    changed.
> 
> Changes in v6:
>  - Fixed a compile error with !CONFIG_X86_MAILBOX_WAKEUP.
>  - Added Reviewed-by tag from Dexuan. Thanks!
> 
> Changes in v5:
>  - None
> 
> Changes in v4:
>  - Updated to use the renamed function acpi_get_mp_wakeup_mailbox_paddr().
>  - Added Reviewed-by tag from Michael. Thanks!
> 
> Changes in v3:
>  - Use the new helper function get_mp_wakeup_mailbox_paddr().
>  - Edited the commit message for clarity.
> 
> Changes in v2:
>  - 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 | 17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
> 
> diff --git a/arch/x86/hyperv/hv_vtl.c b/arch/x86/hyperv/hv_vtl.c
> index 752101544663..2af825f7a447 100644
> --- a/arch/x86/hyperv/hv_vtl.c
> +++ b/arch/x86/hyperv/hv_vtl.c
> @@ -6,6 +6,9 @@
>   *   Saurabh Sengar <ssengar@xxxxxxxxxxxxx>
>   */
>  
> +#include <linux/acpi.h>
> +
> +#include <asm/acpi.h>
>  #include <asm/apic.h>
>  #include <asm/boot.h>
>  #include <asm/desc.h>
> @@ -59,6 +62,18 @@ 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 = acpi_get_mp_wakeup_mailbox_paddr();
> +
> +	return mb_addr && within_page(addr, mb_addr);
> +}
> +
>  void __init hv_vtl_init_platform(void)
>  {
>  	/*
> @@ -71,6 +86,8 @@ 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
>