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[2003:cb:c70b:b400:e20a:6d03:7ac8:f97d]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-438b31d9a39sm58733405e9.31.2025.01.23.03.39.44 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 23 Jan 2025 03:39:46 -0800 (PST) Message-ID: <82d8d3a3-6f06-4904-9d94-6f92bba89dbc@redhat.com> Date: Thu, 23 Jan 2025 12:39:43 +0100 Precedence: bulk X-Mailing-List: linux-arm-msm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: 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: 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, roypat@amazon.co.uk, 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> 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 23.01.25 10:48, Fuad Tabba wrote: > On Wed, 22 Jan 2025 at 22:10, David Hildenbrand wrote: >> >> On 22.01.25 16:27, Fuad Tabba wrote: >>> Make kvm_(read|/write)_guest_page() capable of accessing guest >>> memory for slots that don't have a userspace address, but only if >>> the memory is mappable, which also indicates that it is >>> accessible by the host. >> >> Interesting. So far my assumption was that, for shared memory, user >> space would simply mmap() guest_memdd and pass it as userspace address >> to the same memslot that has this guest_memfd for private memory. >> >> Wouldn't that be easier in the first shot? (IOW, not require this patch >> with the cost of faulting the shared page into the page table on access) > In light of: https://lkml.kernel.org/r/20250117190938.93793-4-imbrenda@linux.ibm.com there can, in theory, be memslots that start at address 0 and have a "valid" mapping. This case is done from the kernel (and on special s390x hardware), though, so it does not apply here at all so far. In practice, getting address 0 as a valid address is unlikely, because the default: $ sysctl vm.mmap_min_addr vm.mmap_min_addr = 65536 usually prohibits it for good reason. > This has to do more with the ABI I had for pkvm and shared memory > implementations, in which you don't need to specify the userspace > address for memory in a guestmem memslot. The issue is there is no > obvious address to map it to. This would be the case in kvm:arm64 for > tracking paravirtualized time, which the userspace doesn't necessarily > need to interact with, but kvm does. So I understand correctly: userspace wouldn't have to mmap it because it is not interested in accessing it, but there is nothing speaking against mmaping it, at least in the first shot. I assume it would not be a private memslot (so far, my understanding is that internal memslots never have a guest_memfd attached). kvm_gmem_create() is only called via KVM_CREATE_GUEST_MEMFD, to be set on user-created memslots. > > 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? 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. 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) -- Cheers, David / dhildenb