linux-kernel - Re: [PATCH v2 1/7] x86/boot: Remove verify_cpu() from secondary_startup

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Date:   Thu, 19 Jan 2023 11:35:06 -0800
From:   "H. Peter Anvin" <hpa@...or.com>
To:     Peter Zijlstra <peterz@...radead.org>, x86@...nel.org,
        Joan Bruguera <joanbrugueram@...il.com>
CC:     linux-kernel@...r.kernel.org, Juergen Gross <jgross@...e.com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        xen-devel <xen-devel@...ts.xenproject.org>,
        Jan Beulich <jbeulich@...e.com>,
        Roger Pau Monne <roger.pau@...rix.com>,
        Kees Cook <keescook@...omium.org>, mark.rutland@....com,
        Andrew Cooper <Andrew.Cooper3@...rix.com>,
        Jörg Rödel <joro@...tes.org>,
        jroedel@...e.de, kirill.shutemov@...ux.intel.com,
        dave.hansen@...el.com, kai.huang@...el.com
Subject: Re: [PATCH v2 1/7] x86/boot: Remove verify_cpu() from secondary_startup_64()

On January 18, 2023 1:45:44 AM PST, Peter Zijlstra <peterz@...radead.org> wrote:
>On Mon, Jan 16, 2023 at 03:25:34PM +0100, Peter Zijlstra wrote:
>> The boot trampolines from trampoline_64.S have code flow like:
>> 
>>   16bit BIOS			SEV-ES				64bit EFI
>> 
>>   trampoline_start()		sev_es_trampoline_start()	trampoline_start_64()
>>     verify_cpu()			  |				|
>>   switch_to_protected:    <---------------'				v
>>        |							pa_trampoline_compat()
>>        v								|
>>   startup_32()		<-----------------------------------------------'
>>        |
>>        v
>>   startup_64()
>>        |
>>        v
>>   tr_start() := head_64.S:secondary_startup_64()
>> 
>> Since AP bringup always goes through the 16bit BIOS path (EFI doesn't
>> touch the APs), there is already a verify_cpu() invocation.
>
>So supposedly TDX/ACPI-6.4 comes in on trampoline_startup64() for APs --
>can any of the TDX capable folks tell me if we need verify_cpu() on
>these?
>
>Aside from checking for LM, it seems to clear XD_DISABLE on Intel and
>force enable SSE on AMD/K7. Surely none of that is needed for these
>shiny new chips?
>
>I mean, I can hack up a patch that adds verify_cpu() to the 64bit entry
>point, but it seems really sad to need that on modern systems.

Sad, perhaps, but really better for orthogonality – fewer special cases.