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

[Borislav Petkov]

On Mon, Jan 24, 2022 at 06:02:02PM +0300, 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>

I wonder what that Reported-by tag means here for this is a feature
patch, not a bug fix or so...

> diff --git a/arch/x86/include/asm/realmode.h b/arch/x86/include/asm/realmode.h
> index 331474b150f1..fd6f6e5b755a 100644
> --- a/arch/x86/include/asm/realmode.h
> +++ b/arch/x86/include/asm/realmode.h
> @@ -25,6 +25,7 @@ struct real_mode_header {
>  	u32	sev_es_trampoline_start;
>  #endif
>  #ifdef CONFIG_X86_64
> +	u32	trampoline_start64;
>  	u32	trampoline_pgd;
>  #endif

Hmm, so there's trampoline_start, sev_es_trampoline_start and
trampoline_start64. If those are mutually exclusive, can we merge them
all into a single trampoline_start?

-- 
Regards/Gruss,
    Boris.

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