linux-kernel - Re: [PATCH v3 2/2] x86/sev: Get the AP jump table address from secrets page

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Date:   Fri, 22 Apr 2022 12:27:11 -0500
From:   Tom Lendacky <thomas.lendacky@....com>
To:     Michael Roth <michael.roth@....com>
Cc:     linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H . Peter Anvin" <hpa@...or.com>,
        Kees Cook <keescook@...omium.org>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Tony Luck <tony.luck@...el.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Venu Busireddy <venu.busireddy@...cle.com>,
        Joerg Roedel <jroedel@...e.de>,
        Tianyu Lan <Tianyu.Lan@...rosoft.com>,
        Brijesh Singh <brijesh.singh@....com>
Subject: Re: [PATCH v3 2/2] x86/sev: Get the AP jump table address from
 secrets page

On 4/22/22 10:40, Michael Roth wrote:
> On Fri, Apr 22, 2022 at 10:15:08AM -0500, Tom Lendacky wrote:
>> On 4/22/22 08:56, Michael Roth wrote:
>>> From: Brijesh Singh <brijesh.singh@....com>
>>>
>>> The GHCB specification section 2.7 states that when SEV-SNP is enabled,
>>> a hypervisor must provide the AP jump table physical address through
>>
>> I missed this on the first version. It's not the hypervisor, but the guest
>> BIOS that directly provides the AP jump table physical address, in our case
>> OVMF sets the address in the SNP secrets page. This allows communication
>> between UEFI/BIOS and OS without hypervisor involvement.
> 
> How about this wording for the commit message?
> 
>    The GHCB specification section 2.7 states that when SEV-SNP is enabled,
>    a guest should not rely on the hypervisor to provide the address of the
>    AP jump table. Instead, if a guest BIOS wants provide an AP jump table,
>    it should record the address in the SNP secrets page so the guest
>    operating system can obtain it directly from there.
> 
>    Fix this on the guest kernel side by having SNP guests use the AP jump
>    table address published in the secrets page rather than issuing a GHCB
>    request to get it.

That sounds good to me.

Thanks,
Tom

> 
>>
>>> the SNP secrets pages.
>>>
>>> Fixes: 0afb6b660a6b ("x86/sev: Use SEV-SNP AP creation to start secondary CPUs")
>>> Signed-off-by: Brijesh Singh <brijesh.singh@....com>
>>> [ mroth: improve error handling when ioremap()/memremap() return NULL ]
>>> [ mroth: don't mix function calls with declarations ]
>>> [ mroth: add missing __init ]
>>> Signed-off-by: Michael Roth <michael.roth@....com>
>>
>> With the commit message change:
>>
>> Reviewed-by: Tom Lendacky <thomas.lendacky@....com>