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[79.242.59.245]) by smtp.gmail.com with ESMTPSA id f5sm3231993wmb.47.2021.08.31.12.12.44 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 31 Aug 2021 12:12:45 -0700 (PDT) Subject: Re: [RFC] KVM: mm: fd-based approach for supporting KVM guest private memory To: Sean Christopherson , Andy Lutomirski Cc: Paolo Bonzini , Vitaly Kuznetsov , Wanpeng Li , Jim Mattson , Joerg Roedel , kvm list , Linux Kernel Mailing List , Borislav Petkov , Andrew Morton , Joerg Roedel , Andi Kleen , David Rientjes , Vlastimil Babka , Tom Lendacky , Thomas Gleixner , "Peter Zijlstra (Intel)" , Ingo Molnar , Varad Gautam , Dario Faggioli , the arch/x86 maintainers , linux-mm@kvack.org, linux-coco@lists.linux.dev, "Kirill A. Shutemov" , "Kirill A . Shutemov" , Sathyanarayanan Kuppuswamy , Dave Hansen , Yu Zhang References: <20210824005248.200037-1-seanjc@google.com> <307d385a-a263-276f-28eb-4bc8dd287e32@redhat.com> <40af9d25-c854-8846-fdab-13fe70b3b279@kernel.org> <73319f3c-6f5e-4f39-a678-7be5fddd55f2@www.fastmail.com> From: David Hildenbrand Organization: Red Hat Message-ID: <949e6d95-266d-0234-3b86-6bd3c5267333@redhat.com> Date: Tue, 31 Aug 2021 21:12:44 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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'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. -- Thanks, David / dhildenb