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[79.242.51.234]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4280d37bb88sm152794745e9.38.2024.07.30.03.17.43 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 30 Jul 2024 03:17:44 -0700 (PDT) Message-ID: <1195d9ce-3f16-4b11-b6d2-88f593a25e0d@redhat.com> Date: Tue, 30 Jul 2024 12:17:42 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH 0/8] Unmapping guest_memfd from Direct Map To: Patrick Roy , "Vlastimil Babka (SUSE)" , seanjc@google.com, pbonzini@redhat.com, akpm@linux-foundation.org, dwmw@amazon.co.uk, rppt@kernel.org Cc: tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com, x86@kernel.org, hpa@zytor.com, willy@infradead.org, graf@amazon.com, derekmn@amazon.com, kalyazin@amazon.com, kvm@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, dmatlack@google.com, tabba@google.com, chao.p.peng@linux.intel.com, xmarcalx@amazon.co.uk References: <20240709132041.3625501-1-roypat@amazon.co.uk> <7e175521-38bb-49f0-b1fb-8820f8708c9c@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-Rspamd-Queue-Id: 96366A0016 X-Stat-Signature: zu16u31gyks6mq6nfoh9jfama1ogetib X-Rspamd-Server: rspam09 X-Rspam-User: X-HE-Tag: 1722334670-198654 X-HE-Meta: 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 sG8qICTl B2n8gvYoQzMiF5JM3vYFN9HnxnVSgeGgJf6mcxIlSI78P4V+UhcHQ8sag5RcKqrbBMjt2fu1FgjhkHlm8jbIVBl9uPwijTGO5Hqr9cpxmqHH2VGZmlAgQ0Xy3BfnWCv4Adyt65SCS7u38RjBgJOaNdAMjxeygkSwlqZDYAU1Zn5EBE9u0rcUmfPrw3C7lRO9LYOSAIbqxjvk6WLDoZHmcdN1J8dxmSGw2snAIIW+AJ8gDckopPC4COFscqtqVXKebDaxrX2VOZfn8+PMqWswSBUhQgSN8Uvx0ycetcHHH/2Bh8ONEkJfkt4t00TEat4t5xOgeHN03gbWIls5gC406ZXlvCS6KmT4dGCBZCdTf8w7Ve7jMLTLfO1X9gZIB+3V+HmnyorueE1oiA22pmd3ZARmZ1UIcv0fqqrrfc3XZM1Y012P9z6+PiBwFNQ== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, 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 26.07.24 08:55, Patrick Roy wrote: > > > On Mon, 2024-07-22 at 13:28 +0100, "Vlastimil Babka (SUSE)" wrote: >>> === Implementation === >>> >>> This patch series introduces a new flag to the `KVM_CREATE_GUEST_MEMFD` >>> to remove its pages from the direct map when they are allocated. When >>> trying to run a guest from such a VM, we now face the problem that >>> without either userspace or kernelspace mappings of guest_memfd, KVM >>> cannot access guest memory to, for example, do MMIO emulation of access >>> memory used to guest/host communication. We have multiple options for >>> solving this when running non-CoCo VMs: (1) implement a TDX-light >>> solution, where the guest shares memory that KVM needs to access, and >>> relies on paravirtual solutions where this is not possible (e.g. MMIO), >>> (2) have KVM use userspace mappings of guest_memfd (e.g. a >>> memfd_secret-style solution), or (3) dynamically reinsert pages into the >>> direct map whenever KVM wants to access them. >>> >>> This RFC goes for option (3). Option (1) is a lot of overhead for very >>> little gain, since we are not actually constrained by a physical >>> inability to access guest memory (e.g. we are not in a TDX context where >>> accesses to guest memory cause a #MC). Option (2) has previously been >>> rejected [1]. >> >> Do the pages have to have the same address when they are temporarily mapped? >> Wouldn't it be easier to do something similar to kmap_local_page() used for >> HIMEM? I.e. you get a temporary kernel mapping to do what's needed, but it >> doesn't have to alter the shared directmap. >> >> Maybe that was already discussed somewhere as unsuitable but didn't spot it >> here. > > For what I had prototyped here, there's no requirement to have the pages > mapped at the same address (I remember briefly looking at memremap to > achieve the temporary mappings, but since that doesnt work for normal > memory, I gave up on that path). However, I think guest_memfd is moving > into a direction where ranges marked as "in-place shared" (e.g. those > that are temporarily reinserted into the direct map in this RFC) should > be able to be GUP'd [1]. I think for that the direct map entries would > need to be present, right? Yes, we'd allow GUP. Of course, one could think of a similar extension like secretmem that would allow shared memory to get mapped into user page tables but would disallow any GUP on (shared) guest_memfd memory. -- Cheers, David / dhildenb