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[2003:cb:c721:8100:177e:1983:5478:64ec]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-432dab721d7sm61558775e9.9.2024.11.15.09.10.34 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 15 Nov 2024 09:10:36 -0800 (PST) Message-ID: Date: Fri, 15 Nov 2024 18:10:33 +0100 Precedence: bulk X-Mailing-List: linux-s390@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: 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 Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 15.11.24 17:59, Patrick Roy wrote: > > > On Tue, 2024-11-12 at 14:52 +0000, David Hildenbrand wrote: >> On 12.11.24 15:40, Patrick Roy wrote: >>> 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. > > Well, if we start on-demand mm-local mapping the things we want to > access, we're back in TLB flush hell, no? At least the on-demand mapping shouldn't require a TLB flush? Only "unmapping" if we want to restrict the size of a "mapped pool" of restricted size. Anyhow, this would be a pure optimization, to avoid the expense of mapping everything, when in practice you'd like not access most of it. (my theory, happy to be told I'm wrong :) ) > And we can't know > ahead-of-time what needs to be mapped, so everything would need to be > mapped (unless we do something like mm-local mapping a page on first > access, and then just never unmapping it again, under the assumption > that establishing the mapping won't be expensive) Right, the whole problem is that we don't know that upfront. > >>> >>> 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 :) > > I've talked to Fares internally, and it seems that generally doing > mm-local mappings of guest memory would work for us. We also figured out > what the "interrupt problem" is, namely that if we receive an interrupt > while executing in a context that has mm-local mappings available, those > mappings will continue to be available while the interrupt is being > handled. Isn't that likely also the case with secretmem where we removed the directmap, but have an effective per-mm mapping in the (user-space portion) of the page table? > I'm talking to my security folks to see how much of a concern > this is for the speculation hardening we're trying to achieve. Will keep > you in the loop there :) Thanks! -- Cheers, David / dhildenb