From: David Hildenbrand <david@redhat.com>
To: Sean Christopherson <seanjc@google.com>
Cc: Andy Lutomirski <luto@kernel.org>,
Paolo Bonzini <pbonzini@redhat.com>,
Vitaly Kuznetsov <vkuznets@redhat.com>,
Wanpeng Li <wanpengli@tencent.com>,
Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>,
kvm list <kvm@vger.kernel.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Borislav Petkov <bp@alien8.de>,
Andrew Morton <akpm@linux-foundation.org>,
Joerg Roedel <jroedel@suse.de>, Andi Kleen <ak@linux.intel.com>,
David Rientjes <rientjes@google.com>,
Vlastimil Babka <vbabka@suse.cz>,
Tom Lendacky <thomas.lendacky@amd.com>,
Thomas Gleixner <tglx@linutronix.de>,
"Peter Zijlstra (Intel)" <peterz@infradead.org>,
Ingo Molnar <mingo@redhat.com>,
Varad Gautam <varad.gautam@suse.com>,
Dario Faggioli <dfaggioli@suse.com>,
the arch/x86 maintainers <x86@kernel.org>,
linux-mm@kvack.org, linux-coco@lists.linux.dev,
"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
"Kirill A . Shutemov" <kirill@shutemov.name>,
Sathyanarayanan Kuppuswamy
<sathyanarayanan.kuppuswamy@linux.intel.com>,
Dave Hansen <dave.hansen@intel.com>,
Yu Zhang <yu.c.zhang@linux.intel.com>
Subject: Re: [RFC] KVM: mm: fd-based approach for supporting KVM guest private memory
Date: Wed, 1 Sep 2021 09:51:17 +0200 [thread overview]
Message-ID: <a21fc9cf-0775-2c70-0ad6-61bb1363e2d0@redhat.com> (raw)
In-Reply-To: <YS6U1u/p+nOXVEfS@google.com>
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
next prev parent reply other threads:[~2021-09-01 7:51 UTC|newest]
Thread overview: 71+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-08-24 0:52 [RFC] KVM: mm: fd-based approach for supporting KVM guest private memory Sean Christopherson
2021-08-24 10:48 ` Yu Zhang
2021-08-26 0:35 ` Sean Christopherson
2021-08-26 13:23 ` Yu Zhang
2021-08-26 10:15 ` David Hildenbrand
2021-08-26 17:05 ` Andy Lutomirski
2021-08-26 21:26 ` David Hildenbrand
2021-08-27 18:24 ` Andy Lutomirski
2021-08-27 22:28 ` Sean Christopherson
2021-08-31 19:12 ` David Hildenbrand
2021-08-31 20:45 ` Sean Christopherson
2021-09-01 7:51 ` David Hildenbrand [this message]
2021-08-27 2:31 ` Yu Zhang
2021-08-31 19:08 ` David Hildenbrand
2021-08-31 20:01 ` Andi Kleen
2021-08-31 20:15 ` David Hildenbrand
2021-08-31 20:39 ` Andi Kleen
2021-09-01 3:34 ` Yu Zhang
2021-09-01 4:53 ` Andy Lutomirski
2021-09-01 7:12 ` Tian, Kevin
2021-09-01 10:24 ` Yu Zhang
2021-09-01 16:07 ` Andy Lutomirski
2021-09-01 16:27 ` David Hildenbrand
2021-09-02 8:34 ` Yu Zhang
2021-09-02 8:44 ` David Hildenbrand
2021-09-02 11:02 ` Yu Zhang
2021-09-02 8:19 ` Yu Zhang
2021-09-02 18:41 ` Andy Lutomirski
2021-09-07 1:33 ` Yan Zhao
2021-09-02 9:27 ` Joerg Roedel
2021-09-02 18:41 ` Andy Lutomirski
2021-09-02 18:57 ` Sean Christopherson
2021-09-02 19:07 ` Dave Hansen
2021-09-02 20:42 ` Andy Lutomirski
2021-08-27 22:18 ` Sean Christopherson
2021-08-31 19:07 ` David Hildenbrand
2021-08-31 21:54 ` Sean Christopherson
2021-09-01 8:09 ` David Hildenbrand
2021-09-01 15:54 ` Andy Lutomirski
2021-09-01 16:16 ` David Hildenbrand
2021-09-01 17:09 ` Andy Lutomirski
2021-09-01 16:18 ` James Bottomley
2021-09-01 16:22 ` David Hildenbrand
2021-09-01 16:31 ` James Bottomley
2021-09-01 16:37 ` David Hildenbrand
2021-09-01 16:45 ` James Bottomley
2021-09-01 17:08 ` David Hildenbrand
2021-09-01 17:50 ` Sean Christopherson
2021-09-01 17:53 ` David Hildenbrand
2021-09-01 17:08 ` Andy Lutomirski
2021-09-01 17:13 ` James Bottomley
2021-09-02 10:18 ` Joerg Roedel
2021-09-01 18:24 ` Andy Lutomirski
2021-09-01 19:26 ` Dave Hansen
2021-09-07 15:00 ` Tom Lendacky
2021-09-01 4:58 ` Andy Lutomirski
2021-09-01 7:49 ` David Hildenbrand
2021-09-02 18:47 ` Kirill A. Shutemov
2021-09-02 20:33 ` Sean Christopherson
2021-09-03 19:14 ` Kirill A. Shutemov
2021-09-03 19:15 ` Andy Lutomirski
2021-09-10 17:18 ` Kirill A. Shutemov
2021-09-15 19:58 ` Chao Peng
2021-09-15 13:51 ` David Hildenbrand
2021-09-15 14:29 ` Kirill A. Shutemov
2021-09-15 14:59 ` David Hildenbrand
2021-09-15 15:35 ` David Hildenbrand
2021-09-15 20:04 ` Kirill A. Shutemov
2021-09-15 14:11 ` Kirill A. Shutemov
2021-09-16 7:36 ` Chao Peng
2021-09-16 9:24 ` Paolo Bonzini
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