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[2003:cb:c70b:b400:e20a:6d03:7ac8:f97d]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-38bf3221761sm19134760f8f.22.2025.01.23.06.18.35 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 23 Jan 2025 06:18:37 -0800 (PST) Message-ID: Date: Thu, 23 Jan 2025 15:18:34 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH v1 2/9] KVM: guest_memfd: Add guest_memfd support to kvm_(read|/write)_guest_page() To: Patrick Roy , Fuad Tabba Cc: kvm@vger.kernel.org, linux-arm-msm@vger.kernel.org, linux-mm@kvack.org, pbonzini@redhat.com, chenhuacai@kernel.org, mpe@ellerman.id.au, anup@brainfault.org, paul.walmsley@sifive.com, palmer@dabbelt.com, aou@eecs.berkeley.edu, seanjc@google.com, viro@zeniv.linux.org.uk, brauner@kernel.org, willy@infradead.org, akpm@linux-foundation.org, xiaoyao.li@intel.com, yilun.xu@intel.com, chao.p.peng@linux.intel.com, jarkko@kernel.org, amoorthy@google.com, dmatlack@google.com, yu.c.zhang@linux.intel.com, isaku.yamahata@intel.com, mic@digikod.net, vbabka@suse.cz, vannapurve@google.com, ackerleytng@google.com, mail@maciej.szmigiero.name, michael.roth@amd.com, wei.w.wang@intel.com, liam.merwick@oracle.com, isaku.yamahata@gmail.com, kirill.shutemov@linux.intel.com, suzuki.poulose@arm.com, steven.price@arm.com, quic_eberman@quicinc.com, quic_mnalajal@quicinc.com, quic_tsoni@quicinc.com, quic_svaddagi@quicinc.com, quic_cvanscha@quicinc.com, quic_pderrin@quicinc.com, quic_pheragu@quicinc.com, catalin.marinas@arm.com, james.morse@arm.com, yuzenghui@huawei.com, oliver.upton@linux.dev, maz@kernel.org, will@kernel.org, qperret@google.com, keirf@google.com, shuah@kernel.org, hch@infradead.org, jgg@nvidia.com, rientjes@google.com, jhubbard@nvidia.com, fvdl@google.com, hughd@google.com, jthoughton@google.com References: <20250122152738.1173160-1-tabba@google.com> <20250122152738.1173160-3-tabba@google.com> <82d8d3a3-6f06-4904-9d94-6f92bba89dbc@redhat.com> 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: C29F840003 X-Stat-Signature: ttcphd441idefo5s3jq5xyx4zfsejw89 X-Rspam-User: X-Rspamd-Server: rspam12 X-HE-Tag: 1737641923-555974 X-HE-Meta: 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 pVcf9mRN 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000040, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: >>> >>> That said, we could always have a userspace address dedicated to >>> mapping shared locations, and use that address when the necessity >>> arises. Or we could always require that memslots have a userspace >>> address, even if not used. I don't really have a strong preference. >> >> So, the simpler version where user space would simply mmap guest_memfd >> to provide the address via userspace_addr would at least work for the >> use case of paravirtualized time? > > fwiw, I'm currently prototyping something like this for x86 (although > not by putting the gmem address into userspace_addr, but by adding a new > field to memslots, so that memory attributes continue working), based on > what we talked about at the last guest_memfd sync meeting (the whole > "how to get MMIO emulation working for non-CoCo VMs in guest_memfd" > story). Yes, I recall that discussion. Can you elaborate why the separate field is required to keep memory attributes working? (could it be sorted out differently, by reusing userspace_addr?). > So I guess if we're going down this route for x86, maybe it > makes sense to do the same on ARM, for consistency? > >> It would get rid of the immediate need for this patch and patch #4 to >> get it flying. >> >> >> One interesting question is: when would you want shared memory in >> guest_memfd and *not* provide it as part of the same memslot. > > In my testing of non-CoCo gmem VMs on ARM, I've been able to get quite > far without giving KVM a way to internally access shared parts of gmem - > it's why I was probing Fuad for this simplified series, because > KVM_SW_PROTECTED_VM + mmap (for loading guest kernel) is enough to get a > working non-CoCo VM on ARM (although I admittedly never looked at clocks > inside the guest - maybe that's one thing that breaks if KVM can't > access gmem. How to guest and host agree on the guest memory range > used to exchange paravirtual timekeeping information? Could that exchange > be intercepted in userspace, and set to shared via memory attributes (e.g. > placed outside gmem)? That's the route I'm going down the paravirtual > time on x86). Sounds reasonable to me. > >> One nice thing about the mmap might be that access go via user-space >> page tables: E.g., __kvm_read_guest_page can just access the memory >> without requiring the folio lock and an additional temporary folio >> reference on every access -- it's handled implicitly via the mapcount. >> >> (of course, to map the page we still need that once on the fault path) > > Doing a direct map access in kvm_{read,write}_guest() and friends will > also get tricky if guest_memfd folios ever don't have direct map > entries. On-demand restoration is painful, both complexity and > performance wise [1], while going through a userspace mapping of > guest_memfd would "just work". Indeed. -- Cheers, David / dhildenb