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

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Message-ID: <Y8e/yKgVZgbqgvAG@hirez.programming.kicks-ass.net>
Date:   Wed, 18 Jan 2023 10:45:44 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     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>,
        "H. Peter Anvin" <hpa@...or.com>, 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 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.