linux-kernel - Re: [patch 00/37] cpu/hotplug, x86: Reworked parallel CPU bringup

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Date:   Tue, 25 Apr 2023 22:07:47 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Andrew Cooper <andrew.cooper3@...rix.com>,
        Paul Menzel <pmenzel@...gen.mpg.de>,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        David Woodhouse <dwmw2@...radead.org>,
        Brian Gerst <brgerst@...il.com>,
        Arjan van de Veen <arjan@...ux.intel.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Paul McKenney <paulmck@...nel.org>,
        Tom Lendacky <thomas.lendacky@....com>,
        Oleksandr Natalenko <oleksandr@...alenko.name>,
        "Guilherme G. Piccoli" <gpiccoli@...lia.com>,
        Piotr Gorski <lucjan.lucjanov@...il.com>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Usama Arif <usama.arif@...edance.com>,
        Jürgen Groß <jgross@...e.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        xen-devel@...ts.xenproject.org,
        Russell King <linux@...linux.org.uk>,
        Arnd Bergmann <arnd@...db.de>,
        linux-arm-kernel@...ts.infradead.org,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, Guo Ren <guoren@...nel.org>,
        linux-csky@...r.kernel.org,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        linux-mips@...r.kernel.org,
        "James E. J. Bottomley" <James.Bottomley@...senpartnership.com>,
        Helge Deller <deller@....de>, linux-parisc@...r.kernel.org,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        linux-riscv@...ts.infradead.org,
        Mark Rutland <mark.rutland@....com>,
        Sabin Rapan <sabrapan@...zon.com>
Subject: Re: [patch 00/37] cpu/hotplug, x86: Reworked parallel CPU bringup

On Thu, Apr 20 2023 at 17:57, Thomas Gleixner wrote:
> On Thu, Apr 20 2023 at 07:51, Sean Christopherson wrote:
> Something like the completely untested below should just work whatever
> APIC ID the BIOS decided to dice.
>
> That might just work on SEV too without that GHCB muck, but what do I
> know.

It does not.

RDMSR(X2APIC_ID) is trapped via #VC which cannot be handled at that
point. Unfortunately the GHCB protocol does not provide a RDMSR
mechanism similar to the CPUID mechanism. Neither does the secure
firmware enforce CPUID(0xb):APICID to real APIC ID consistency.

So the hypervisor can dice the APIC IDs as long as they are consistent
with the provided ACPI/MADT table.

So no parallel startup for SEV for now.

Thanks,

        tglx