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

Ricardo Neri
2025-05-24T00:26:16Z

On Tue, May 20, 2025 at 01:35:02AM +0000, Michael Kelley wrote:
> From: Ricardo Neri <ricardo.neri-calderon@linux.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@linux.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@outlook.com>

Thank you very much for your review!