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Thu, 22 May 2025 00:47:08 -0700 (PDT) MIME-Version: 1.0 References: <56e17793-efa2-44ea-9491-3217a059d3f3@redhat.com> In-Reply-To: <56e17793-efa2-44ea-9491-3217a059d3f3@redhat.com> From: Fuad Tabba Date: Thu, 22 May 2025 08:46:31 +0100 X-Gm-Features: AX0GCFt-cFyX0KMOfcn0To-snisLLtuWCe8otqaGFWA41BF0Srz0CyrM4W2ydEg Message-ID: Subject: Re: [PATCH v9 09/17] KVM: x86/mmu: Handle guest page faults for guest_memfd with shared memory To: David Hildenbrand Cc: Ackerley Tng , 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 Content-Type: text/plain; charset="UTF-8" X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 8AB66120002 X-Stat-Signature: ehh8bipzno6nhhqguxn57dkyc95qjmw8 X-Rspam-User: X-HE-Tag: 1747900029-672917 X-HE-Meta: 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 1mi9C+GQ DBsc85/Bvnm84MTTatwbrs29g33zwnTnW7gy/aiTaoCfLuzxin3Lbo62vV8IZcWWbZjQa6G79QpXcws2p8OxC8W4yDDnoxaGMXtKTHXrQeEmvXJ7v7rH9izeVRU3teWPwwT3mETOvYQLlQRD2ttwBQvzLbIZjbCf8iRlBVKKXjKr02V7SCeYREP5NVdeXAn1FSRNSIl+67OCSDxETlo0EkQUrXq3IRLpy/LNZpkL5uEBZ91n6Yexy0Vs/h+iv+aXV8YY7xg2AIp/79Cbx61l0FUewZ6Z54a61xetyDHNiF3IpWADXww6KpWCRf81cvvAYdOMfE1S/mbjPXOvCaD/y0M6HCFIoOD5KyBKVCQWtS8Is2Lr71XQlK1TxObk3mtUsbx5sVd7XbwCqZFs= 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: Hi David, On Thu, 22 May 2025 at 08:16, David Hildenbrand wrote: > > > >>> + * 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. I'm not sure we can do that. The comment in kvm_gmem_bind() on dropping the reference to the file explains why: https://elixir.bootlin.com/linux/v6.14.7/source/virt/kvm/guest_memfd.c#L526 I think the best thing is to track whether a slot supports shared memory inside struct kvm_memory_slot::struct gmem. Thanks, /fuad > -- > Cheers, > > David / dhildenb >