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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: kvm@vger.kernel.org, David Hildenbrand , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Chao Peng , linux-riscv@lists.infradead.org, Isaku Yamahata , Marc Zyngier , Huacai Chen , Xiaoyao Li , "Matthew Wilcox \(Oracle\)" , Wang , Fuad Tabba , Yu Zhang , Maciej Szmigiero , Albert Ou , Vlastimil Babka , Michael Roth , Ackerley Tng , Alexander Viro , Paul Walmsley , kvmarm@lists.linux.dev, linux-arm-kernel@lists.infradead.org, =?UTF-8?B?TWlja2HDq2wgU2FsYcO8?= =?UTF-8?Q?n?= , Isaku Yamahata , Christian Brauner , Quentin Perret , linux-mips@vger.kernel.org, Oliver Upton , Jarkko Sakkinen , Palmer Dabbelt , "Kirill A . Shutemov" , kvm-riscv@lists.infradead.org, Anup Patel , linux-fsdevel@vger.kernel.org, Liam Merwick , Andrew Morton , Vishal Annapurve , linuxppc-dev@lists.ozlabs.org, Xu Yilun , Anish Moorthy Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" On 10/31/23 23:39, David Matlack wrote: >>> Maybe can you sketch out how you see this proposal being extensible to >>> using guest_memfd for shared mappings? >> For in-place conversions, e.g. pKVM, no additional guest_memfd is needed. What's >> missing there is the ability to (safely) mmap() guest_memfd, e.g. KVM needs to >> ensure there are no outstanding references when converting back to private. >> >> For TDX/SNP, assuming we don't find a performant and robust way to do in-place >> conversions, a second fd+offset pair would be needed. > Is there a way to support non-in-place conversions within a single guest_memfd? For TDX/SNP, you could have a hook from KVM_SET_MEMORY_ATTRIBUTES to guest memory. The hook would invalidate now-private parts if they have a VMA, causing a SIGSEGV/EFAULT if the host touches them. It would forbid mappings from multiple gfns to a single offset of the guest_memfd, because then the shared vs. private attribute would be tied to the offset. This should not be a problem; for example, in the case of SNP, the RMP already requires a single mapping from host physical address to guest physical address. Paolo