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[2003:d8:2f22:2e00:6e71:238a:de9f:e396]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3a35ca889a7sm22248682f8f.72.2025.05.22.00.16.45 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 22 May 2025 00:16:48 -0700 (PDT) Message-ID: <56e17793-efa2-44ea-9491-3217a059d3f3@redhat.com> Date: Thu, 22 May 2025 09:16:45 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v9 09/17] KVM: x86/mmu: Handle guest page faults for guest_memfd with shared memory To: Ackerley Tng Cc: tabba@google.com, 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, 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, pankaj.gupta@amd.com, ira.weiny@intel.com References: 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: D217E14000A X-Stat-Signature: rheouttdtwthcae1g88zyxhkbiyjj96y X-Rspam-User: X-Rspamd-Server: rspam07 X-HE-Tag: 1747898212-325063 X-HE-Meta: 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 XqhoVA6u 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: >>> + * shared (i.e., non-CoCo VMs). >>> + */ >>> static inline bool kvm_mem_is_private(struct kvm *kvm, gfn_t gfn) >>> { >>> - return IS_ENABLED(CONFIG_KVM_GMEM) && >>> - kvm_get_memory_attributes(kvm, gfn) & KVM_MEMORY_ATTRIBUTE_PRIVATE; >>> + struct kvm_memory_slot *slot; >>> + >>> + if (!IS_ENABLED(CONFIG_KVM_GMEM)) >>> + return false; >>> + >>> + slot = gfn_to_memslot(kvm, gfn); >>> + if (kvm_slot_has_gmem(slot) && kvm_gmem_memslot_supports_shared(slot)) { >>> + /* >>> + * For now, memslots only support in-place shared memory if the >>> + * host is allowed to mmap memory (i.e., non-Coco VMs). >>> + */ >> >> Not accurate: there is no in-place conversion support in this series, >> because there is no such itnerface. So the reason is that all memory is >> shared for there VM types? >> > > True that there's no in-place conversion yet. > > In this patch series, guest_memfd memslots support shared memory only > for specific VM types (on x86, that would be KVM_X86_DEFAULT_VM and > KVM_X86_SW_PROTECTED_VMs). > > How about this wording: > > Without conversion support, if the guest_memfd memslot supports shared > memory, all memory must be used as not private (implicitly shared). > LGTM >>> + return false; >>> + } >>> + >>> + return kvm_get_memory_attributes(kvm, gfn) & KVM_MEMORY_ATTRIBUTE_PRIVATE; >>> } >>> #else >>> static inline bool kvm_mem_is_private(struct kvm *kvm, gfn_t gfn) >>> diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c >>> index 2f499021df66..fe0245335c96 100644 >>> --- a/virt/kvm/guest_memfd.c >>> +++ b/virt/kvm/guest_memfd.c >>> @@ -388,6 +388,23 @@ static int kvm_gmem_mmap(struct file *file, struct vm_area_struct *vma) >>> >>> return 0; >>> } >>> + >>> +bool kvm_gmem_memslot_supports_shared(const struct kvm_memory_slot *slot) >>> +{ >>> + struct file *file; >>> + bool ret; >>> + >>> + file = kvm_gmem_get_file((struct kvm_memory_slot *)slot); >>> + if (!file) >>> + return false; >>> + >>> + ret = kvm_gmem_supports_shared(file_inode(file)); >>> + >>> + fput(file); >>> + return ret; >> >> Would it make sense to cache that information in the memslot, to avoid >> the get/put? >> >> We could simply cache when creating the memslot I guess. >> > > When I wrote it I was assuming that to ensure correctness we should > check with guest memfd, like what if someone closed the gmem file in the > middle of the fault path? > > But I guess after the discussion at the last call, since the faulting > process is long and racy, if this check passed and we go to guest memfd > and the file was closed, it would just fail so I guess caching is fine. Yes, that would be my assumption. I mean, we also msut make sure that if the user does something stupid like that, that we won't trigger other undesired code paths (like, suddenly the guest_memfd being !shared). > >> As an alternative ... could we simple get/put when managing the memslot? > > What does a simple get/put mean here? s/simple/simply/ So when we create the memslot, we'd perform the get, and when we destroy the memslot, we'd do the put. Just an idea. -- Cheers, David / dhildenb