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Mon, 21 Jul 2025 10:31:40 -0700 (PDT) Date: Mon, 21 Jul 2025 10:31:38 -0700 In-Reply-To: <20250717162731.446579-20-tabba@google.com> Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20250717162731.446579-1-tabba@google.com> <20250717162731.446579-20-tabba@google.com> Message-ID: Subject: Re: [PATCH v15 19/21] KVM: Introduce the KVM capability KVM_CAP_GMEM_MMAP From: Sean Christopherson To: Fuad Tabba Cc: kvm@vger.kernel.org, linux-arm-msm@vger.kernel.org, linux-mm@kvack.org, kvmarm@lists.linux.dev, pbonzini@redhat.com, chenhuacai@kernel.org, mpe@ellerman.id.au, anup@brainfault.org, paul.walmsley@sifive.com, palmer@dabbelt.com, aou@eecs.berkeley.edu, 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, ackerleytng@google.com, mail@maciej.szmigiero.name, david@redhat.com, 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="us-ascii" On Thu, Jul 17, 2025, Fuad Tabba wrote: > Introduce the new KVM capability KVM_CAP_GMEM_MMAP. This capability > signals to userspace that a KVM instance supports host userspace mapping > of guest_memfd-backed memory. > > The availability of this capability is determined per architecture, and > its enablement for a specific guest_memfd instance is controlled by the > GUEST_MEMFD_FLAG_MMAP flag at creation time. > > Update the KVM API documentation to detail the KVM_CAP_GMEM_MMAP > capability, the associated GUEST_MEMFD_FLAG_MMAP, and provide essential > information regarding support for mmap in guest_memfd. > > Reviewed-by: David Hildenbrand > Reviewed-by: Gavin Shan > Reviewed-by: Shivank Garg > Signed-off-by: Fuad Tabba > --- > Documentation/virt/kvm/api.rst | 9 +++++++++ > include/uapi/linux/kvm.h | 1 + > virt/kvm/kvm_main.c | 4 ++++ > 3 files changed, 14 insertions(+) > > diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst > index 43ed57e048a8..5169066b53b2 100644 > --- a/Documentation/virt/kvm/api.rst > +++ b/Documentation/virt/kvm/api.rst > @@ -6407,6 +6407,15 @@ most one mapping per page, i.e. binding multiple memory regions to a single > guest_memfd range is not allowed (any number of memory regions can be bound to > a single guest_memfd file, but the bound ranges must not overlap). > > +When the capability KVM_CAP_GMEM_MMAP is supported, the 'flags' field supports > +GUEST_MEMFD_FLAG_MMAP. Setting this flag on guest_memfd creation enables mmap() > +and faulting of guest_memfd memory to host userspace. > + > +When the KVM MMU performs a PFN lookup to service a guest fault and the backing > +guest_memfd has the GUEST_MEMFD_FLAG_MMAP set, then the fault will always be > +consumed from guest_memfd, regardless of whether it is a shared or a private > +fault. > + > See KVM_SET_USER_MEMORY_REGION2 for additional details. > > 4.143 KVM_PRE_FAULT_MEMORY > diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h > index 3beafbf306af..698dd407980f 100644 > --- a/include/uapi/linux/kvm.h > +++ b/include/uapi/linux/kvm.h > @@ -960,6 +960,7 @@ struct kvm_enable_cap { > #define KVM_CAP_ARM_EL2 240 > #define KVM_CAP_ARM_EL2_E2H0 241 > #define KVM_CAP_RISCV_MP_STATE_RESET 242 > +#define KVM_CAP_GMEM_MMAP 243 KVM_CAP_GUEST_MEMFD_MMAP please. I definitely don't want "gmem" in any of the uAPI. > struct kvm_irq_routing_irqchip { > __u32 irqchip; > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c > index 46bddac1dacd..f1ac872e01e9 100644 > --- a/virt/kvm/kvm_main.c > +++ b/virt/kvm/kvm_main.c > @@ -4916,6 +4916,10 @@ static int kvm_vm_ioctl_check_extension_generic(struct kvm *kvm, long arg) > #ifdef CONFIG_KVM_GMEM > case KVM_CAP_GUEST_MEMFD: > return !kvm || kvm_arch_supports_gmem(kvm); > +#endif > +#ifdef CONFIG_KVM_GMEM_SUPPORTS_MMAP > + case KVM_CAP_GMEM_MMAP: As alluded to in my feedback in patch 2, this should be inside CONFIG_KVM_GUEST_MEMFD. > + return !kvm || kvm_arch_supports_gmem_mmap(kvm); > #endif > default: > break; > -- > 2.50.0.727.gbf7dc18ff4-goog >