linux-kernel - Re: [PATCH v16 8/8] x86/smpboot: Allow parallel bringup for SEV-ES

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Date:   Thu, 23 Mar 2023 08:32:49 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Borislav Petkov <bp@...en8.de>,
        Usama Arif <usama.arif@...edance.com>
Cc:     tglx@...utronix.de, kim.phillips@....com, brgerst@...il.com,
        piotrgorski@...hyos.org, oleksandr@...alenko.name,
        arjan@...ux.intel.com, mingo@...hat.com,
        dave.hansen@...ux.intel.com, hpa@...or.com, x86@...nel.org,
        pbonzini@...hat.com, paulmck@...nel.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        rcu@...r.kernel.org, mimoja@...oja.de, hewenliang4@...wei.com,
        thomas.lendacky@....com, seanjc@...gle.com, pmenzel@...gen.mpg.de,
        fam.zheng@...edance.com, punit.agrawal@...edance.com,
        simon.evans@...edance.com, liangma@...ngbit.com,
        gpiccoli@...lia.com, Sabin Rapan <sabrapan@...zon.com>
Subject: Re: [PATCH v16 8/8] x86/smpboot: Allow parallel bringup for SEV-ES

On Wed, 2023-03-22 at 23:47 +0100, Borislav Petkov wrote:
> 
> +.Luse_sev_cpuid_0b:
> +       call sev_get_cpuid_0b
> +       test %eax, %eax
> +       jz 1f
> +       movl %eax, %edx
> +       jmp .Lsetup_AP
> +

Can this code path never be taken for bringing up CPU0 even after a
hotplug? I'd rather have -1 as the error indication.

>  .Luse_cpuid_0b:
>         mov     $0x0B, %eax
>         xorl    %ecx, %ecx
> diff --git a/arch/x86/kernel/sev.c b/arch/x86/kernel/sev.c
> index ce371f62167b..96ff63cb5622 100644
> --- a/arch/x86/kernel/sev.c
> +++ b/arch/x86/kernel/sev.c
> @@ -1142,6 +1142,24 @@ void snp_set_wakeup_secondary_cpu(void)
>         apic->wakeup_secondary_cpu = wakeup_cpu_via_vmgexit;
>  }
>  
> +u32 sev_get_cpuid_0b(void)
> +{
> +       u32 eax, edx;
> +
> +       /* Request CPUID 0xB_EDX through GHCB protocol */
> +       native_wrmsr(MSR_AMD64_SEV_ES_GHCB,
> +                    (GHCB_CPUID_REQ_EDX << 30) | GHCB_MSR_CPUID_REQ,
> +                    0xb);
> +       VMGEXIT();
> +
> +       native_rdmsr(MSR_AMD64_SEV_ES_GHCB, eax, edx);
> +
> +       if ((eax & GHCB_MSR_INFO_MASK) == GHCB_MSR_CPUID_RESP)
> +               return edx;
> +
> +       return 0;
> +}
> +
>  int __init sev_es_setup_ap_jump_table(struct real_mode_header *rmh)
>  {

Perhaps put this in head64.c so it gets built with -pg when needed? And
then you'll spot that the other functions in there are marked __head to
put them in .head.text, and maybe find some other stuff to cargo-cult
to make it safe to run C code that early...

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