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([2a01:599:922:1046:67aa:7b35:f780:c8fc]) by smtp.gmail.com with ESMTPSA id 4fb4d7f45d1cf-5dc1863b68fsm1311747a12.46.2025.01.24.06.44.58 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 24 Jan 2025 06:45:03 -0800 (PST) Message-ID: <14803371-ee5f-4a13-91ff-29b9e05ec9f7@redhat.com> Date: Fri, 24 Jan 2025 15:44:57 +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: 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> <5b2949bf-ab8b-46d4-9daf-71fe3e20b0c8@amazon.co.uk> 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 16:22, Patrick Roy wrote: > On Thu, 2025-01-23 at 14:18 +0000, David Hildenbrand wrote: >>>>> >>>>> 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?). > > The scenario I ran into was that within the same memslots, I wanted some > gfns to be backed by guest_memfd, and others by traditional memory, so > that KVM can GUP some parts of guest memory even if guest_memfd itself > is direct map removed. Just summarizing what we discussed yesterday: GUP will not be allowed if the direct map was removed, and paravirt time on x86 uses GUP to access the guest page right now. > > It actually also has to do with paravirtual time, but on x86. Here, the > guest chooses where in guest memory the clock structure is placed via an > MSR write (so I can't a priori use a traditional memslot, like we can on > ARM). KVM internally wants to GUP the hva that corresponds to the gfn > the guest chooses, but if the hva is in a mapping of direct map removed > gmem, that won't work. And as discussed here, Sean raised that these hypervisor updates happen rarely. In the first step, it might be good enough to just update using the user-space page table mappings (user access), to avoid messing with kmap/direct-map reinstalling. With that in place, it looks like that user space could simply mmap guest_memfd and provide the mmaped area "ordinarily" as the "shared memory" part of the guest_memfd -- using userspace_address. GUP would fail, bit paravirt time would not be using GUP. So we could handle the guest_memfd mmap just like on other architectures, without the need for other memslot fields. Please correct me if I misunderstood something :) -- Cheers, David / dhildenb