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[2003:d8:2f02:2900:f54f:bad7:c5f4:9404]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-43f235a5d57sm188361965e9.34.2025.04.14.13.06.44 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 14 Apr 2025 13:06:45 -0700 (PDT) Message-ID: <6121b93b-6390-49e9-82db-4ed3a6797898@redhat.com> Date: Mon, 14 Apr 2025 22:06:43 +0200 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: [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> 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've been thinking long about this, and was wondering if we should instead >> clean up the code to decouple the "private" from gmem handling first. >> > > Thank you for making this suggestion more concrete, I like the renaming! > Thanks for the fast feedback! >> I know, this was already discussed a couple of times, but faking that >> shared memory is private looks odd. >> >> I played with the code to star cleaning this up. I ended up with the following >> gmem-terminology cleanup patches (not even compile tested) >> >> KVM: rename CONFIG_KVM_GENERIC_PRIVATE_MEM to CONFIG_KVM_GENERIC_GMEM_POPULATE >> KVM: rename CONFIG_KVM_PRIVATE_MEM to CONFIG_KVM_GMEM >> KVM: rename kvm_arch_has_private_mem() to kvm_arch_supports_gmem() >> KVM: x86: rename kvm->arch.has_private_mem to kvm->arch.supports_gmem >> KVM: rename kvm_slot_can_be_private() to kvm_slot_has_gmem() > > Perhaps zooming into this [1] can clarify a lot. In > kvm_mmu_max_mapping_level(), it was > > bool is_private = kvm_slot_has_gmem(slot) && kvm_mem_is_private(kvm, gfn); > > and now it is > > bool is_gmem = kvm_slot_has_gmem(slot) && kvm_mem_from_gmem(kvm, gfn); > > Is this actually saying that the mapping level is to be fully determined > from lpage_info as long as this memslot has gmem and With this change in particular I was not quite sure what to do, maybe it should stay specific to private memory only? But yeah the ideas was that kvm_mem_from_gmem() would express: (a) if guest_memfd only supports private memory, it would translate to kvm_mem_is_private() -> no change. (b) with guest_memfd having support for shared memory (+ support being enabled!), it would only rely on the slot, not gfn information. Because it will all be consumed from guest_memfd. This hunk was missing diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h index d9616ee6acc70..cdcd7ac091b5c 100644 --- a/include/linux/kvm_host.h +++ b/include/linux/kvm_host.h @@ -2514,6 +2514,12 @@ static inline bool kvm_mem_is_private(struct kvm *kvm, gfn_t gfn) } #endif /* CONFIG_KVM_GENERIC_MEMORY_ATTRIBUTES */ +static inline bool kvm_mem_from_gmem(struct kvm *kvm, gfn_t gfn) +{ + /* For now, only private memory gets consumed from guest_memfd. */ + return kvm_mem_is_private(kvm, gfn); +} + > > 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? -- Cheers, David / dhildenb