linux-kernel - Re: [PATCH v8 08/40] x86/sev: Check the vmpl level

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Date:   Tue, 4 Jan 2022 09:23:25 -0600
From:   Brijesh Singh <brijesh.singh@....com>
To:     Borislav Petkov <bp@...en8.de>,
        Tom Lendacky <thomas.lendacky@....com>
Cc:     brijesh.singh@....com, Mikolaj Lisik <lisik@...gle.com>,
        Venu Busireddy <venu.busireddy@...cle.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-efi@...r.kernel.org, platform-driver-x86@...r.kernel.org,
        linux-coco@...ts.linux.dev, linux-mm@...ck.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Joerg Roedel <jroedel@...e.de>,
        "H. Peter Anvin" <hpa@...or.com>, Ard Biesheuvel <ardb@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Jim Mattson <jmattson@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Sergio Lopez <slp@...hat.com>, Peter Gonda <pgonda@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        David Rientjes <rientjes@...gle.com>,
        Dov Murik <dovmurik@...ux.ibm.com>,
        Tobin Feldman-Fitzthum <tobin@....com>,
        Michael Roth <michael.roth@....com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Andi Kleen <ak@...ux.intel.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        tony.luck@...el.com, marcorr@...gle.com,
        sathyanarayanan.kuppuswamy@...ux.intel.com
Subject: Re: [PATCH v8 08/40] x86/sev: Check the vmpl level



On 12/20/21 12:10 PM, Borislav Petkov wrote:
> On Fri, Dec 17, 2021 at 04:33:02PM -0600, Tom Lendacky wrote:
>>>>>> +      * There is no straightforward way to query the current VMPL level. The
>>>>>> +      * simplest method is to use the RMPADJUST instruction to change a page
>>>>>> +      * permission to a VMPL level-1, and if the guest kernel is launched at
>>>>>> +      * a level <= 1, then RMPADJUST instruction will return an error.
>>>>> Perhaps a nit. When you say "level <= 1", do you mean a level lower than or
>>>>> equal to 1 semantically, or numerically?
>>>
>>> Its numerically, please see the AMD APM vol 3.
>>
>> Actually it is not numerically...  if it was numerically, then 0 <= 1 would
>> return an error, but VMPL0 is the highest permission level.
> 
> Just write in that comment exactly what this function does:
> 
> "RMPADJUST modifies RMP permissions of a lesser-privileged (numerically
> higher) privilege level. Here, clear the VMPL1 permission mask of the
> GHCB page. If the guest is not running at VMPL0, this will fail.
> 
> If the guest is running at VMP0, it will succeed. Even if that operation
> modifies permission bits, it is still ok to do currently because Linux
> SNP guests are supported only on VMPL0 so VMPL1 or higher permission
> masks changing is a don't-care."
> 
> and then everything is clear wrt numbering, privilege, etc.
> 
> Ok?
> 

Noted.

thanks