linux-kernel - Re: [PATCH Part2 v5 00/45] Add AMD Secure Nested Paging (SEV-SNP) Hypervisor Support

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Date:   Mon, 15 Nov 2021 12:58:54 -0600
From:   Brijesh Singh <brijesh.singh@....com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     brijesh.singh@....com, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...el.com>,
        Peter Gonda <pgonda@...gle.com>, 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>,
        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 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>, tony.luck@...el.com,
        marcorr@...gle.com, sathyanarayanan.kuppuswamy@...ux.intel.com
Subject: Re: [PATCH Part2 v5 00/45] Add AMD Secure Nested Paging (SEV-SNP)
 Hypervisor Support



On 11/15/21 12:44 PM, Sean Christopherson wrote:
> On Mon, Nov 15, 2021, Brijesh Singh wrote:
>>
>> On 11/12/21 2:37 PM, Sean Christopherson wrote:
>>> This is the direction KVM TDX support is headed, though it's obviously still a WIP.
>>>
>>
>> Just curious, in this approach, how do you propose handling the host
>> kexec/kdump? If a kexec/kdump occurs while the VM is still active, the new
>> kernel will encounter the #PF (RMP violation) because some pages are still
>> marked 'private' in the RMP table.
> 
> There are two basic options: a) eagerly purge the RMP or b) lazily fixup the RMP
> on #PF.  Either approach can be made to work.  I'm not opposed to fixing up the RMP
> on #PF in the kexec/kdump case, I'm opposed to blindly updating the RMP on _all_
> RMP #PFs, i.e. the kernel should modify the RMP if and only if it knows that doing
> so is correct.  E.g. a naive lazy-fixup solution would be to track which pages have
> been sanitized and adjust the RMP on #PF to a page that hasn't yet been sanitized.
> 

Yap, I think option #a will require the current kernel to iterate 
through the entire memory and make it shared before booting the kexec 
kernel. It may bring another ask to track the guest private/shared on 
the host to minimize the iterations.

thanks