linux-kernel - Re: [PATCH Part2 v5 21/45] KVM: SVM: Make AVIC backing, VMSA and VMCB memory allocation SNP safe

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Date:   Thu, 23 Sep 2021 17:23:55 -0500
From:   Brijesh Singh <brijesh.singh@....com>
To:     "Dr. David Alan Gilbert" <dgilbert@...hat.com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-coco@...ts.linux.dev, linux-mm@...ck.org,
        linux-crypto@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Joerg Roedel <jroedel@...e.de>,
        Tom Lendacky <thomas.lendacky@....com>,
        "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>,
        Wanpeng Li <wanpengli@...cent.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>,
        Borislav Petkov <bp@...en8.de>,
        Michael Roth <michael.roth@....com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Andi Kleen <ak@...ux.intel.com>, tony.luck@...el.com,
        marcorr@...gle.com, sathyanarayanan.kuppuswamy@...ux.intel.com
Subject: Re: [PATCH Part2 v5 21/45] KVM: SVM: Make AVIC backing, VMSA and VMCB
 memory allocation SNP safe


On 9/23/21 1:39 PM, Dr. David Alan Gilbert wrote:
> * Brijesh Singh (brijesh.singh@....com) wrote:
>> On 9/22/21 1:55 PM, Dr. David Alan Gilbert wrote:
>>> * Brijesh Singh (brijesh.singh@....com) wrote:
>>>> Implement a workaround for an SNP erratum where the CPU will incorrectly
>>>> signal an RMP violation #PF if a hugepage (2mb or 1gb) collides with the
>>>> RMP entry of a VMCB, VMSA or AVIC backing page.
>>>>
>>>> When SEV-SNP is globally enabled, the CPU marks the VMCB, VMSA, and AVIC
>>>> backing   pages as "in-use" in the RMP after a successful VMRUN.  This is
>>>> done for _all_ VMs, not just SNP-Active VMs.
>>> Can you explain what 'globally enabled' means?
>> This means that SNP is enabled in  host SYSCFG_MSR.Snp=1. Once its
>> enabled then RMP checks are enforced.
>>
>>
>>> Or more specifically, can we trip this bug on public hardware that has
>>> the SNP enabled in the bios, but no SNP init in the host OS?
>> Enabling the SNP support on host is 3 step process:
>>
>> step1 (bios): reserve memory for the RMP table.
>>
>> step2 (host): initialize the RMP table memory, set the SYSCFG msr to
>> enable the SNP feature
>>
>> step3 (host): call the SNP_INIT to initialize the SNP firmware (this is
>> needed only if you ever plan to launch SNP guest from this host).
>>
>> The "SNP globally enabled" means the step 1 to 2. The RMP checks are
>> enforced as soon as step 2 is completed.
> So I think that means we don't need to backport this to older kernels
> that don't know about SNP but might run on SNP enabled hardware (1), since
> those kernels won't do step2.

Correct.

thanks