Re: [PATCHv2 16/29] x86/boot: Add a trampoline for booting APs via firmware handoff

[Thomas Gleixner]

On Mon, Jan 24 2022 at 18:02, Kirill A. Shutemov wrote:

> From: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
>
> Historically, x86 platforms have booted secondary processors (APs)
> using INIT followed by the start up IPI (SIPI) messages. In regular
> VMs, this boot sequence is supported by the VMM emulation. But such a
> wakeup model is fatal for secure VMs like TDX in which VMM is an
> untrusted entity. To address this issue, a new wakeup model was added
> in ACPI v6.4, in which firmware (like TDX virtual BIOS) will help boot
> the APs. More details about this wakeup model can be found in ACPI
> specification v6.4, the section titled "Multiprocessor Wakeup Structure".
>
> Since the existing trampoline code requires processors to boot in real
> mode with 16-bit addressing, it will not work for this wakeup model
> (because it boots the AP in 64-bit mode). To handle it, extend the
> trampoline code to support 64-bit mode firmware handoff. Also, extend
> IDT and GDT pointers to support 64-bit mode hand off.
>
> There is no TDX-specific detection for this new boot method. The kernel
> will rely on it as the sole boot method whenever the new ACPI structure
> is present.
>
> The ACPI table parser for the MADT multiprocessor wake up structure and
> the wakeup method that uses this structure will be added by the following
> patch in this series.
>
> Reported-by: Kai Huang <kai.huang@xxxxxxxxx>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> Reviewed-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>
> Reviewed-by: Dan Williams <dan.j.williams@xxxxxxxxx>
> Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@xxxxxxxxxxxxxxx>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>

Reviewed-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>