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[2003:d8:2f02:2900:f54f:bad7:c5f4:9404]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4404352a536sm38056165e9.1.2025.04.16.05.53.22 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 16 Apr 2025 05:53:24 -0700 (PDT) Message-ID: <91c2f8ec-ca03-4368-a220-bac639a18938@redhat.com> Date: Wed, 16 Apr 2025 14:53:21 +0200 Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v7 4/9] KVM: guest_memfd: Handle in-place shared memory as guest_memfd backed memory To: Ackerley Tng , Fuad Tabba , kvm@vger.kernel.org, linux-arm-msm@vger.kernel.org, linux-mm@kvack.org Cc: 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, isaku.yamahata@intel.com, mic@digikod.net, vbabka@suse.cz, vannapurve@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, peterx@redhat.com References: <20250318161823.4005529-1-tabba@google.com> <20250318161823.4005529-5-tabba@google.com> <8ebc66ae-5f37-44c0-884b-564a65467fe4@redhat.com> <6121b93b-6390-49e9-82db-4ed3a6797898@redhat.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 >> > > I looked a little deeper and got help from James Houghton on > understanding this too. Great :) > > Specifically for the usage of kvm_mem_is_private() in > kvm_mmu_max_mapping_level(), the intention there is probably to skip > querying userspace page tables in __kvm_mmu_max_mapping_level() since > private memory will never be faulted into userspace, hence no need to > check. > > Hence kvm_mem_is_private() there is really meant to query the > private-ness of the gfn rather than just whether kvm_mem_from_gmem(). > > But then again, if kvm_mem_from_gmem(), guest_memfd should be queried > for max_mapping_level. guest_memfd would know, for both private and > shared memory, what page size the page was split to, and what level > it was faulted as. (Exception: if/when guest_memfd supports THP, > depending on how that is done, querying userspace page tables might be > necessary to determine the max_mapping_level) Okay, so I assume my intuition was right: if we know we can go via the guest_memfd also for !private memory, then probably no need to consult the page tables. Let's discuss that tomorrow in the meeting. > >>> >>> A. this specific gfn is backed by gmem, or >>> B. if the specific gfn is private? >>> >>> I noticed some other places where kvm_mem_is_private() is left as-is >>> [2], is that intentional? Are you not just renaming but splitting out >>> the case two cases A and B? >> >> That was the idea, yes. >> >> If we get a private fault and !kvm_mem_is_private(), or a shared fault and >> kvm_mem_is_private(), then we should handle it like today. >> >> That is the kvm_mmu_faultin_pfn() case, where we >> >> if (fault->is_private != kvm_mem_is_private(kvm, fault->gfn)) { >> kvm_mmu_prepare_memory_fault_exit(vcpu, fault); >> return -EFAULT; >> } >> >> which can be reached by arch/x86/kvm/svm/svm.c:npf_interception() >> >> if (sev_snp_guest(vcpu->kvm) && (error_code & PFERR_GUEST_ENC_MASK)) >> error_code |= PFERR_PRIVATE_ACCESS; >> >> In summary: the memory attribute mismatch will be handled as is, but not how >> we obtain the gfn. >> >> At least that was the idea (-issues in the commit). >> >> What are your thoughts about that direction? > > I still like the renaming. :) > > I looked into kvm_mem_is_private() and I believe it has the following > uses: > > 1. Determining max_mapping_level (kvm_mmu_max_mapping_level() and > friends) > 2. Querying the kernel's record of private/shared state, which is used > to handle (a) mismatch between fault->private and kernel's record > (handling implicit conversions) (b) how to prefaulting pages (c) > determining how to fault in KVM_X86_SW_PROTECTED_VMs > > So perhaps we could leave kvm_mem_is_private() as not renamed, but as > part of the series introducing mmap and conversions > (CONFIG_KVM_GMEM_SHARED_MEM), we should also have kvm_mem_is_private() > query guest_memfd for shareability status, and perhaps > kvm_mmu_max_mapping_level() could query guest_memfd for page size (after > splitting, etc). > Right, that's why I opted to leave kvm_mem_is_private() here and really only indicate if memory is *actually* private. > IIUC the maximum mapping level is determined by these factors: > > 1. Attribute granularity (lpage_info) > 2. Page size (guest_memfd for guest_memfd backed memory) > 3. Size of mapping in host page table (for non-guest_memfd backed > memory, and important for THP if/when/depending on how guest_memfd > supports THP) Right, once private+shared will come from guest_memfd, then likely 3 does not apply anymore. See my reply to Patrick regarding that. Thanks! -- Cheers, David / dhildenb