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Thu, 02 Nov 2023 09:03:44 -0700 (PDT) Date: Thu, 2 Nov 2023 09:03:42 -0700 In-Reply-To: <6642c379-1023-4716-904f-4bbf076744c2@redhat.com> Mime-Version: 1.0 References: <20231027182217.3615211-1-seanjc@google.com> <20231027182217.3615211-17-seanjc@google.com> <6642c379-1023-4716-904f-4bbf076744c2@redhat.com> Message-ID: Subject: Re: [PATCH v13 16/35] KVM: Add KVM_CREATE_GUEST_MEMFD ioctl() for guest-specific backing memory From: Sean Christopherson To: Paolo Bonzini Cc: David Matlack , Marc Zyngier , Oliver Upton , Huacai Chen , Michael Ellerman , Anup Patel , Paul Walmsley , Palmer Dabbelt , Albert Ou , Alexander Viro , Christian Brauner , "Matthew Wilcox (Oracle)" , Andrew Morton , kvm@vger.kernel.org, linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev, linux-mips@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, kvm-riscv@lists.infradead.org, linux-riscv@lists.infradead.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Xiaoyao Li , Xu Yilun , Chao Peng , Fuad Tabba , Jarkko Sakkinen , Anish Moorthy , Yu Zhang , Isaku Yamahata , "=?utf-8?Q?Micka=C3=ABl_Sala=C3=BCn?=" , Vlastimil Babka , Vishal Annapurve , Ackerley Tng , Maciej Szmigiero , David Hildenbrand , Quentin Perret , Michael Roth , Wang , Liam Merwick , Isaku Yamahata , "Kirill A . Shutemov" Content-Type: text/plain; charset="us-ascii" X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: 559DBC0189 X-Stat-Signature: nhdr7b3mhpqd7x6m47s4xq8u86rrpx9m X-Rspam-User: X-HE-Tag: 1698941025-592427 X-HE-Meta: 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 0fEoALWv 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 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 Thu, Nov 02, 2023, Paolo Bonzini wrote: > 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. I don't see how this can work. It's not a M:1 scenario (where M is multiple gfns), it's a 1:N scenario (wheren N is multiple offsets). The *gfn* doesn't change on a conversion, what needs to change to do non-in-place conversion is the pfn, which is effectively the guest_memfd+offset pair. So yes, we *could* support non-in-place conversions within a single guest_memfd, but it would require a second offset, at which point it makes sense to add a second file descriptor as well. Userspace could still use a single guest_memfd instance, i.e. pass in the same file descriptor but different offsets.