Date:   Wed, 1 Sep 2021 09:51:17 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm list <kvm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Borislav Petkov <bp@...en8.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Joerg Roedel <jroedel@...e.de>,
        Andi Kleen <ak@...ux.intel.com>,
        David Rientjes <rientjes@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Tom Lendacky <thomas.lendacky@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Varad Gautam <varad.gautam@...e.com>,
        Dario Faggioli <dfaggioli@...e.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        linux-mm@...ck.org, linux-coco@...ts.linux.dev,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Sathyanarayanan Kuppuswamy 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Yu Zhang <yu.c.zhang@...ux.intel.com>
Subject: Re: [RFC] KVM: mm: fd-based approach for supporting KVM guest private
 memory

On 31.08.21 22:45, Sean Christopherson wrote:
> On Tue, Aug 31, 2021, David Hildenbrand wrote:
>> On 28.08.21 00:28, Sean Christopherson wrote:
>>> On Fri, Aug 27, 2021, Andy Lutomirski wrote:
>>>>
>>>> On Thu, Aug 26, 2021, at 2:26 PM, David Hildenbrand wrote:
>>>>> On 26.08.21 19:05, Andy Lutomirski wrote:
>>>>
>>>>>> Oof.  That's quite a requirement.  What's the point of the VMA once all
>>>>>> this is done?
>>>>>
>>>>> You can keep using things like mbind(), madvise(), ... and the GUP code
>>>>> with a special flag might mostly just do what you want. You won't have
>>>>> to reinvent too many wheels on the page fault logic side at least.
>>>
>>> Ya, Kirill's RFC more or less proved a special GUP flag would indeed Just Work.
>>> However, the KVM page fault side of things would require only a handful of small
>>> changes to send private memslots down a different path.  Compared to the rest of
>>> the enabling, it's quite minor.
>>>
>>> The counter to that is other KVM architectures would need to learn how to use the
>>> new APIs, though I suspect that there will be a fair bit of arch enabling regardless
>>> of what route we take.
>>>
>>>> You can keep calling the functions.  The implementations working is a
>>>> different story: you can't just unmap (pte_numa-style or otherwise) a private
>>>> guest page to quiesce it, move it with memcpy(), and then fault it back in.
>>>
>>> Ya, I brought this up in my earlier reply.  Even the initial implementation (without
>>> real NUMA support) would likely be painful, e.g. the KVM TDX RFC/PoC adds dedicated
>>> logic in KVM to handle the case where NUMA balancing zaps a _pinned_ page and then
>>> KVM fault in the same pfn.  It's not thaaat ugly, but it's arguably more invasive
>>> to KVM's page fault flows than a new fd-based private memslot scheme.
>>
>> I might have a different mindset, but less code churn doesn't necessarily
>> translate to "better approach".
> 
> I wasn't referring to code churn.  By "invasive" I mean number of touchpoints in
> KVM as well as the nature of the touchpoints.  E.g. poking into how KVM uses
> available bits in its shadow PTEs and adding multiple checks through KVM's page
> fault handler, versus two callbacks to get the PFN and page size.
> 
>> I'm certainly not pushing for what I proposed (it's a rough, broken sketch).
>> I'm much rather trying to come up with alternatives that try solving the
>> same issue, handling the identified requirements.
>>
>> I have a gut feeling that the list of requirements might not be complete
>> yet. For example, I wonder if we have to protect against user space
>> replacing private pages by shared pages or punishing random holes into the
>> encrypted memory fd.
> 
> Replacing a private page with a shared page for a given GFN is very much a
> requirement as it's expected behavior for all VMM+guests when converting guest
> memory between shared and private.
> 
> Punching holes is a sort of optional requirement.  It's a "requirement" in that
> it's allowed if the backing store supports such a behavior, optional in that
> support wouldn't be strictly necessary and/or could come with constraints.  The
> expected use case is that host userspace would punch a hole to free unreachable
> private memory, e.g. after the corresponding GFN(s) is converted to shared, so
> that it doesn't consume 2x memory for the guest.
> 

Okay, that matches my understanding then. I was rather thinking about 
"what happens if we punch a hole where private memory was not converted 
to shared yet". AFAIU, we will simply crash the guest then.

-- 
Thanks,

David / dhildenb

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