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[175.34.62.5]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-23506bc8db2sm113392715ad.41.2025.06.04.23.41.00 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 04 Jun 2025 23:41:19 -0700 (PDT) Message-ID: Date: Thu, 5 Jun 2025 16:40:58 +1000 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 v10 08/16] KVM: guest_memfd: Allow host to map guest_memfd pages To: 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, 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 References: <20250527180245.1413463-1-tabba@google.com> <20250527180245.1413463-9-tabba@google.com> Content-Language: en-US From: Gavin Shan In-Reply-To: <20250527180245.1413463-9-tabba@google.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Hi Fuad, On 5/28/25 4:02 AM, Fuad Tabba wrote: > This patch enables support for shared memory in guest_memfd, including > mapping that memory at the host userspace. This support is gated by the > configuration option KVM_GMEM_SHARED_MEM, and toggled by the guest_memfd > flag GUEST_MEMFD_FLAG_SUPPORT_SHARED, which can be set when creating a > guest_memfd instance. > > Co-developed-by: Ackerley Tng > Signed-off-by: Ackerley Tng > Signed-off-by: Fuad Tabba > --- > arch/x86/include/asm/kvm_host.h | 10 ++++ > arch/x86/kvm/x86.c | 3 +- > include/linux/kvm_host.h | 13 ++++++ > include/uapi/linux/kvm.h | 1 + > virt/kvm/Kconfig | 5 ++ > virt/kvm/guest_memfd.c | 81 +++++++++++++++++++++++++++++++++ > 6 files changed, 112 insertions(+), 1 deletion(-) > > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h > index 709cc2a7ba66..ce9ad4cd93c5 100644 > --- a/arch/x86/include/asm/kvm_host.h > +++ b/arch/x86/include/asm/kvm_host.h > @@ -2255,8 +2255,18 @@ void kvm_configure_mmu(bool enable_tdp, int tdp_forced_root_level, > > #ifdef CONFIG_KVM_GMEM > #define kvm_arch_supports_gmem(kvm) ((kvm)->arch.supports_gmem) > + > +/* > + * CoCo VMs with hardware support that use guest_memfd only for backing private > + * memory, e.g., TDX, cannot use guest_memfd with userspace mapping enabled. > + */ > +#define kvm_arch_supports_gmem_shared_mem(kvm) \ > + (IS_ENABLED(CONFIG_KVM_GMEM_SHARED_MEM) && \ > + ((kvm)->arch.vm_type == KVM_X86_SW_PROTECTED_VM || \ > + (kvm)->arch.vm_type == KVM_X86_DEFAULT_VM)) > #else > #define kvm_arch_supports_gmem(kvm) false > +#define kvm_arch_supports_gmem_shared_mem(kvm) false > #endif > > #define kvm_arch_has_readonly_mem(kvm) (!(kvm)->arch.has_protected_state) > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c > index 035ced06b2dd..2a02f2457c42 100644 > --- a/arch/x86/kvm/x86.c > +++ b/arch/x86/kvm/x86.c > @@ -12718,7 +12718,8 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type) > return -EINVAL; > > kvm->arch.vm_type = type; > - kvm->arch.supports_gmem = (type == KVM_X86_SW_PROTECTED_VM); > + kvm->arch.supports_gmem = > + type == KVM_X86_DEFAULT_VM || type == KVM_X86_SW_PROTECTED_VM; > /* Decided by the vendor code for other VM types. */ > kvm->arch.pre_fault_allowed = > type == KVM_X86_DEFAULT_VM || type == KVM_X86_SW_PROTECTED_VM; > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h > index 80371475818f..ba83547e62b0 100644 > --- a/include/linux/kvm_host.h > +++ b/include/linux/kvm_host.h > @@ -729,6 +729,19 @@ static inline bool kvm_arch_supports_gmem(struct kvm *kvm) > } > #endif > > +/* > + * Returns true if this VM supports shared mem in guest_memfd. > + * > + * Arch code must define kvm_arch_supports_gmem_shared_mem if support for > + * guest_memfd is enabled. > + */ > +#if !defined(kvm_arch_supports_gmem_shared_mem) && !IS_ENABLED(CONFIG_KVM_GMEM) > +static inline bool kvm_arch_supports_gmem_shared_mem(struct kvm *kvm) > +{ > + return false; > +} > +#endif > + > #ifndef kvm_arch_has_readonly_mem > static inline bool kvm_arch_has_readonly_mem(struct kvm *kvm) > { > diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h > index b6ae8ad8934b..c2714c9d1a0e 100644 > --- a/include/uapi/linux/kvm.h > +++ b/include/uapi/linux/kvm.h > @@ -1566,6 +1566,7 @@ struct kvm_memory_attributes { > #define KVM_MEMORY_ATTRIBUTE_PRIVATE (1ULL << 3) > > #define KVM_CREATE_GUEST_MEMFD _IOWR(KVMIO, 0xd4, struct kvm_create_guest_memfd) > +#define GUEST_MEMFD_FLAG_SUPPORT_SHARED (1ULL << 0) > > struct kvm_create_guest_memfd { > __u64 size; > diff --git a/virt/kvm/Kconfig b/virt/kvm/Kconfig > index 559c93ad90be..df225298ab10 100644 > --- a/virt/kvm/Kconfig > +++ b/virt/kvm/Kconfig > @@ -128,3 +128,8 @@ config HAVE_KVM_ARCH_GMEM_PREPARE > config HAVE_KVM_ARCH_GMEM_INVALIDATE > bool > depends on KVM_GMEM > + > +config KVM_GMEM_SHARED_MEM > + select KVM_GMEM > + bool > + prompt "Enable support for non-private (shared) memory in guest_memfd" > diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c > index 6db515833f61..5d34712f64fc 100644 > --- a/virt/kvm/guest_memfd.c > +++ b/virt/kvm/guest_memfd.c > @@ -312,7 +312,81 @@ static pgoff_t kvm_gmem_get_index(struct kvm_memory_slot *slot, gfn_t gfn) > return gfn - slot->base_gfn + slot->gmem.pgoff; > } > > +static bool kvm_gmem_supports_shared(struct inode *inode) > +{ > + u64 flags; > + > + if (!IS_ENABLED(CONFIG_KVM_GMEM_SHARED_MEM)) > + return false; > + > + flags = (u64)inode->i_private; > + > + return flags & GUEST_MEMFD_FLAG_SUPPORT_SHARED; > +} > + > + > +#ifdef CONFIG_KVM_GMEM_SHARED_MEM > +static vm_fault_t kvm_gmem_fault_shared(struct vm_fault *vmf) > +{ > + struct inode *inode = file_inode(vmf->vma->vm_file); > + struct folio *folio; > + vm_fault_t ret = VM_FAULT_LOCKED; > + > + folio = kvm_gmem_get_folio(inode, vmf->pgoff); > + if (IS_ERR(folio)) { > + int err = PTR_ERR(folio); > + > + if (err == -EAGAIN) > + return VM_FAULT_RETRY; > + > + return vmf_error(err); > + } > + > + if (WARN_ON_ONCE(folio_test_large(folio))) { > + ret = VM_FAULT_SIGBUS; > + goto out_folio; > + } > + > + if (!folio_test_uptodate(folio)) { > + clear_highpage(folio_page(folio, 0)); > + kvm_gmem_mark_prepared(folio); > + } > + > + vmf->page = folio_file_page(folio, vmf->pgoff); > + > +out_folio: > + if (ret != VM_FAULT_LOCKED) { > + folio_unlock(folio); > + folio_put(folio); > + } > + > + return ret; > +} > + > +static const struct vm_operations_struct kvm_gmem_vm_ops = { > + .fault = kvm_gmem_fault_shared, > +}; > + > +static int kvm_gmem_mmap(struct file *file, struct vm_area_struct *vma) > +{ > + if (!kvm_gmem_supports_shared(file_inode(file))) > + return -ENODEV; > + > + if ((vma->vm_flags & (VM_SHARED | VM_MAYSHARE)) != > + (VM_SHARED | VM_MAYSHARE)) { > + return -EINVAL; > + } > + > + vma->vm_ops = &kvm_gmem_vm_ops; > + > + return 0; > +} > +#else > +#define kvm_gmem_mmap NULL > +#endif /* CONFIG_KVM_GMEM_SHARED_MEM */ > + > static struct file_operations kvm_gmem_fops = { > + .mmap = kvm_gmem_mmap, > .open = generic_file_open, > .release = kvm_gmem_release, > .fallocate = kvm_gmem_fallocate, > @@ -463,6 +537,9 @@ int kvm_gmem_create(struct kvm *kvm, struct kvm_create_guest_memfd *args) > u64 flags = args->flags; > u64 valid_flags = 0; > It seems there is an uncovered corner case, which exists in current code (not directly caused by this patch): After .mmap is hooked, the address space (inode->i_mapping) is exposed to user space for futher requests like madvise().madvise(MADV_COLLAPSE) can potentially collapse the pages to a huge page (folio) with the following assumptions. It's not the expected behavior since huge page isn't supported yet. - CONFIG_READ_ONLY_THP_FOR_FS = y - the folios in the pagecache have been fully populated, it can be done by kvm_gmem_fallocate() or kvm_gmem_get_pfn(). - mmap(0x00000f0100000000, ..., MAP_FIXED_NOREPLACE) on the guest-memfd, and then do madvise(buf, size, MADV_COLLAPSE). sys_madvise do_madvise madvise_do_behavior madvise_vma_behavior madvise_collapse thp_vma_allowable_order file_thp_enabled // need to return false to bail from the path earlier at least hpage_collapse_scan_file collapse_pte_mapped_thp The fix would be to increase inode->i_writecount using allow_write_access() in __kvm_gmem_create() to break the check done by file_thp_enabled(). diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c index 0cd12f94958b..fe706c9f21cf 100644 --- a/virt/kvm/guest_memfd.c +++ b/virt/kvm/guest_memfd.c @@ -502,6 +502,7 @@ static int __kvm_gmem_create(struct kvm *kvm, loff_t size, u64 flags) } file->f_flags |= O_LARGEFILE; + allow_write_access(file); > + if (kvm_arch_supports_gmem_shared_mem(kvm)) > + valid_flags |= GUEST_MEMFD_FLAG_SUPPORT_SHARED; > + > if (flags & ~valid_flags) > return -EINVAL; > > @@ -501,6 +578,10 @@ int kvm_gmem_bind(struct kvm *kvm, struct kvm_memory_slot *slot, > offset + size > i_size_read(inode)) > goto err; > > + if (kvm_gmem_supports_shared(inode) && > + !kvm_arch_supports_gmem_shared_mem(kvm)) > + goto err; > + > filemap_invalidate_lock(inode->i_mapping); > > start = offset >> PAGE_SHIFT; Thanks, Gavin