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[2003:cb:c739:8e00:7a46:1b8c:8b13:d3d]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-432b05e5b96sm220710455e9.41.2024.11.12.06.52.59 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 12 Nov 2024 06:53:00 -0800 (PST) Message-ID: Date: Tue, 12 Nov 2024 15:52:58 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH v3 0/6] Direct Map Removal for guest_memfd To: Patrick Roy , tabba@google.com, quic_eberman@quicinc.com, seanjc@google.com, pbonzini@redhat.com, jthoughton@google.com, ackerleytng@google.com, vannapurve@google.com, rppt@kernel.org Cc: graf@amazon.com, jgowans@amazon.com, derekmn@amazon.com, kalyazin@amazon.com, xmarcalx@amazon.com, linux-mm@kvack.org, corbet@lwn.net, catalin.marinas@arm.com, will@kernel.org, chenhuacai@kernel.org, kernel@xen0n.name, paul.walmsley@sifive.com, palmer@dabbelt.com, aou@eecs.berkeley.edu, hca@linux.ibm.com, gor@linux.ibm.com, agordeev@linux.ibm.com, borntraeger@linux.ibm.com, svens@linux.ibm.com, gerald.schaefer@linux.ibm.com, tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com, x86@kernel.org, hpa@zytor.com, luto@kernel.org, peterz@infradead.org, rostedt@goodmis.org, mhiramat@kernel.org, mathieu.desnoyers@efficios.com, shuah@kernel.org, kvm@vger.kernel.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, loongarch@lists.linux.dev, linux-riscv@lists.infradead.org, linux-s390@vger.kernel.org, linux-trace-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, faresx@amazon.com References: <20241030134912.515725-1-roypat@amazon.co.uk> <4aa0ccf4-ebbe-4244-bc85-8bc8dcd14e74@redhat.com> <27646c08-f724-49f7-9f45-d03bad500219@amazon.co.uk> <90c9d8c0-814e-4c86-86ef-439cb5552cb6@amazon.co.uk> <10e4d078-3cdb-4d1c-a1a3-80e91b247217@redhat.com> <02f77d32-e2a1-431b-bb67-33d36c06acd3@amazon.co.uk> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: C97991A0019 X-Stat-Signature: qd8u8zo43up78hhfujr4ejundcn3tnr9 X-HE-Tag: 1731423133-573495 X-HE-Meta: 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 dVy+i1sn rKuMzt4y+sfJNgxpVdoME+X55XrvvY/bUnq2wLeAbYjgbhUvs7Wk7tp79k6bhLjrNFB4khwoPG3TjVWh3fMlgLNTegcYEk0yKhekSRpo67jlJFfFUJHfdxRQ7fTz8p7OmTGmloqXjUhPWOMFyuIeNY5ScoHk2AV2L3LHVkWGMsFsoeVVl5dGl9ZTMEt4ls744/mvjkDfDorWdfXU3R0xhlF7Sv4hiwB10SoZ0HCb4X81GXYNlzwVU77MeDATh3m2yzz84zAHgQYvuEjoZIEIHvAes+kIpOLuId59fOMCup29nhOg7LTsqcBrRY6R1Apugw4en4UfqssZrRI2Tu0tZZ/vb+i6KYLczZpa1dRuyH2kaEoFK/UpfdWgyZJsjkMjMibV90uTgqSir1mjRPVHO8YXfJ7Mk6juS2qnNfzxGPeBLZO4lspoMkcJiH38JgbM9N7C1 X-Bogosity: Ham, tests=bogofilter, spamicity=0.002697, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 12.11.24 15:40, Patrick Roy wrote: > > Hi David, > > sorry for the late response, I ended up catching the flu last week and > was out of commission for a while :( > > On Mon, 2024-11-04 at 21:30 +0000, David Hildenbrand wrote: >>>> We talked about shared (faultable) vs. private (unfaultable), and how it >>>> would interact with the directmap patches here. >>>> >>>> As discussed, having private (unfaultable) memory with the direct-map >>>> removed and shared (faultable) memory with the direct-mapping can make >>>> sense for non-TDX/AMD-SEV/... non-CoCo use cases. Not sure about CoCo, >>>> the discussion here seems to indicate that it might currently not be >>>> required. >>>> >>>> So one thing we could do is that shared (faultable) will have a direct >>>> mapping and be gup-able and private (unfaultable) memory will not have a >>>> direct mapping and is, by design, not gup-able.> >>>> Maybe it could make sense to not have a direct map for all guest_memfd >>>> memory, making it behave like secretmem (and it would be easy to >>>> implement)? But I'm not sure if that is really desirable in VM context. >>> >>> This would work for us (in this scenario, the swiotlb areas would be >>> "traditional" memory, e.g. set to shared via mem attributes instead of >>> "shared" inside KVM), it's kinda what I had prototyped in my v1 of this >>> series (well, we'd need to figure out how to get the mappings of gmem >>> back into KVM, since in this setup, short-circuiting it into >>> userspace_addr wouldn't work, unless we banish swiotlb into a different >>> memslot altogether somehow). >> >> Right. > > "right" as in, "yes we could do that"? :p "right" as in "I see how that could work" :) [...] > > I remember talking to someone at some point about whether we could reuse > the proc-local stuff for guest memory, but I cannot remember the outcome > of that discussion... (or maybe I just wanted to have a discussion about > it, but forgot to follow up on that thought?). I guess we wouldn't use > proc-local _allocations_, but rather just set up proc-local mappings of > the gmem allocations that have been removed from the direct map. Yes. And likely only for memory we really access / try access, if possible. > > I'm wondering, where exactly would be the differences to Sean's idea > about messing with the CR3 register inside KVM to temporarily install > page tables that contain all the gmem stuff, conceptually? Wouldn't we > run into the same interrupt problems that Sean foresaw for the CR3 > stuff? (which, admittedly, I still don't quite follow what these are :( > ). I'd need some more details on that. If anything would rely on the direct mapping (from IRQ context?) than ... we obviously cannot remove the direct mapping :) -- Cheers, David / dhildenb