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

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Date:   Fri, 4 Feb 2022 03:27:19 -0800
From:   "Kuppuswamy, Sathyanarayanan" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>
To:     Borislav Petkov <bp@...en8.de>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     tglx@...utronix.de, mingo@...hat.com, dave.hansen@...el.com,
        luto@...nel.org, peterz@...radead.org, aarcange@...hat.com,
        ak@...ux.intel.com, dan.j.williams@...el.com, david@...hat.com,
        hpa@...or.com, jgross@...e.com, jmattson@...gle.com,
        joro@...tes.org, jpoimboe@...hat.com, knsathya@...nel.org,
        pbonzini@...hat.com, sdeep@...are.com, seanjc@...gle.com,
        tony.luck@...el.com, vkuznets@...hat.com, wanpengli@...cent.com,
        x86@...nel.org, linux-kernel@...r.kernel.org,
        Kai Huang <kai.huang@...el.com>
Subject: Re: [PATCHv2 16/29] x86/boot: Add a trampoline for booting APs via
 firmware handoff


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@...el.com>
>>
>> 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@...el.com>
> 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