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

[Kuppuswamy, Sathyanarayanan]

On 2/2/2022 3:27 AM, Borislav Petkov wrote:
> 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...

I think it was added when Sean created the original patch. I don't have the
full history.

Sean, since this is not a bug fix, shall we remove the Reported-by tag?

> > 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?

trampoline_start and sev_es_trampoline_start are not mutually exclusive. 
Both are
used in arch/x86/kernel/sev.c.

arch/x86/kernel/sev.c:560:      startup_ip = 
(u16)(rmh->sev_es_trampoline_start -
arch/x86/kernel/sev.c:561: rmh->trampoline_start);

But trampoline_start64 can be removed and replaced with 
trampoline_start. But using
_*64 suffix makes it clear that is used for 64 bit(CONFIG_X86_64).

Adding it for clarity seems to be fine to me. But if you would prefer 
single variable, we
can remove it. Please let me know.

>
--
Sathyanarayanan Kuppuswamy
Linux Kernel Developer