linux-kernel - Re: [PATCH v7 00/14] KVM: mm: fd-based approach for supporting KVM guest private memory

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Date:   Tue, 23 Aug 2022 19:41:31 +0200
From:   "Gupta, Pankaj" <pankaj.gupta@....com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Chao Peng <chao.p.peng@...ux.intel.com>,
        "Nikunj A. Dadhania" <nikunj@....com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H . Peter Anvin" <hpa@...or.com>,
        Hugh Dickins <hughd@...gle.com>,
        Jeff Layton <jlayton@...nel.org>,
        "J . Bruce Fields" <bfields@...ldses.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Shuah Khan <shuah@...nel.org>, Mike Rapoport <rppt@...nel.org>,
        Steven Price <steven.price@....com>,
        "Maciej S . Szmigiero" <mail@...iej.szmigiero.name>,
        Vlastimil Babka <vbabka@...e.cz>,
        Vishal Annapurve <vannapurve@...gle.com>,
        Yu Zhang <yu.c.zhang@...ux.intel.com>, luto@...nel.org,
        jun.nakajima@...el.com, dave.hansen@...el.com, ak@...ux.intel.com,
        david@...hat.com, aarcange@...hat.com, ddutile@...hat.com,
        dhildenb@...hat.com, Quentin Perret <qperret@...gle.com>,
        Michael Roth <michael.roth@....com>, mhocko@...e.com,
        Muchun Song <songmuchun@...edance.com>, bharata@....com,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linux-kselftest@...r.kernel.org,
        linux-api@...r.kernel.org, linux-doc@...r.kernel.org,
        qemu-devel@...gnu.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v7 00/14] KVM: mm: fd-based approach for supporting KVM
 guest private memory


>>>>> Actually the current version allows you to delay the allocation to a
>>>>> later time (e.g. page fault time) if you don't call fallocate() on the
>>>>> private fd. fallocate() is necessary in previous versions because we
>>>>> treat the existense in the fd as 'private' but in this version we track
>>>>> private/shared info in KVM so we don't rely on that fact from memory
>>>>> backstores.
>>>>
>>>> Does this also mean reservation of guest physical memory with secure
>>>> processor (both for SEV-SNP & TDX) will also happen at page fault time?
>>>>
>>>> Do we plan to keep it this way?
>>>
>>> If you are talking about accepting memory by the guest, it is initiated by
>>> the guest and has nothing to do with page fault time vs fallocate()
>>> allocation of host memory. I mean acceptance happens after host memory
>>> allocation but they are not in lockstep, acceptance can happen much later.
>>
>> No, I meant reserving guest physical memory range from hypervisor e.g with
>> RMPUpdate for SEV-SNP or equivalent at TDX side (PAMTs?).
> 
> As proposed, RMP/PAMT updates will occur in the fault path, i.e. there is no way
> for userspace to pre-map guest memory.
> 
> I think the best approach is to turn KVM_TDX_INIT_MEM_REGION into a generic
> vCPU-scoped ioctl() that allows userspace to pre-map guest memory.  Supporting
> initializing guest private memory with a source page can be implemented via a
> flag.  That also gives KVM line of sight to in-place "conversion", e.g. another
> flag could be added to say that the dest is also the source.

Questions to clarify *my* understanding here:

- Do you suggest to use KVM_TDX_INIT_MEM_REGION into a generic ioctl to
   pre-map guest private memory in addition to initialize the payload
   (in-place encryption or just copy page to guest private memory)?

- Want to clarify "pre-map": Are you suggesting to use the ioctl
   to avoid the RMP/PAMT registration at guest page fault time? instead
   pre-map guest private memory i.e to allocate and do RMP/PAMT
   registration before running the actual guest vCPU's?

Thanks,
Pankaj

> 
> The TDX and SNP restrictions would then become addition restrictions on when
> initializing with a source is allowed (and VMs that don't have guest private
> memory wouldn't allow the flag at all).
>