On Tue, 2025-05-13 at 17:34 +0100, 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 <ackerleytng@xxxxxxxxxx>
> Signed-off-by: Ackerley Tng <ackerleytng@xxxxxxxxxx>
> Signed-off-by: Fuad Tabba <tabba@xxxxxxxxxx>
> ---
> arch/x86/include/asm/kvm_host.h | 10 ++++
> include/linux/kvm_host.h | 13 +++++
> include/uapi/linux/kvm.h | 1 +
> virt/kvm/Kconfig | 5 ++
> virt/kvm/guest_memfd.c | 88 +++++++++++++++++++++++++++++++++
> 5 files changed, 117 insertions(+)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 709cc2a7ba66..f72722949cae 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_vm_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))
I forgot what we ended up deciding wrt "allow guest_memfd usage for default VMs
on x86" in the call two weeks ago, but if we want to do that as part of this
series, then this also needs
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 12433b1e755b..904b15c678d6 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -12716,7 +12716,7 @@ 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_SW_PROTECTED_VM || type == KVM_X86_DEFAULT_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;
and with that I was able to run my firecracker tests on top of this patch
series with X86_DEFAULT_VM. But I did wonder about this define in
x86/include/asm/kvm_host.h:
/* SMM is currently unsupported for guests with guest_memfd (esp private) memory. */
# define kvm_arch_nr_memslot_as_ids(kvm) (kvm_arch_supports_gmem(kvm) ? 1 : 2)
which I'm not really sure what to make of, but which I think means enabling
guest_memfd for X86_DEFAULT_VM isn't as straight-forward as the above diff :/
Best,
Patrick
> #else
> #define kvm_arch_supports_gmem(kvm) false
> +#define kvm_arch_vm_supports_gmem_shared_mem(kvm) false
> #endif
>
> #define kvm_arch_has_readonly_mem(kvm) (!(kvm)->arch.has_protected_state)
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index ae70e4e19700..2ec89c214978 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_vm_supports_gmem_shared_mem if support for
> + * guest_memfd is enabled.
> + */
> +#if !defined(kvm_arch_vm_supports_gmem_shared_mem) && !IS_ENABLED(CONFIG_KVM_GMEM)
> +static inline bool kvm_arch_vm_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..9857022a0f0c 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 (1UL << 0)
>
> struct kvm_create_guest_memfd {
> __u64 size;
> diff --git a/virt/kvm/Kconfig b/virt/kvm/Kconfig
> index 559c93ad90be..f4e469a62a60 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 "Enables in-place shared memory for guest_memfd"
> diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
> index 6db515833f61..8e6d1866b55e 100644
> --- a/virt/kvm/guest_memfd.c
> +++ b/virt/kvm/guest_memfd.c
> @@ -312,7 +312,88 @@ static pgoff_t kvm_gmem_get_index(struct kvm_memory_slot *slot, gfn_t gfn)
> return gfn - slot->base_gfn + slot->gmem.pgoff;
> }
>
> +#ifdef CONFIG_KVM_GMEM_SHARED_MEM
> +
> +static bool kvm_gmem_supports_shared(struct inode *inode)
> +{
> + uint64_t flags = (uint64_t)inode->i_private;
> +
> + return flags & GUEST_MEMFD_FLAG_SUPPORT_SHARED;
> +}
> +
> +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;
> +
> + filemap_invalidate_lock_shared(inode->i_mapping);
> +
> + folio = kvm_gmem_get_folio(inode, vmf->pgoff);
> + if (IS_ERR(folio)) {
> + int err = PTR_ERR(folio);
> +
> + if (err == -EAGAIN)
> + ret = VM_FAULT_RETRY;
> + else
> + ret = vmf_error(err);
> +
> + goto out_filemap;
> + }
> +
> + if (folio_test_hwpoison(folio)) {
> + ret = VM_FAULT_HWPOISON;
> + goto out_folio;
> + }
> +
> + 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);
> + }
> +
> +out_filemap:
> + filemap_invalidate_unlock_shared(inode->i_mapping);
> +
> + 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 +544,9 @@ int kvm_gmem_create(struct kvm *kvm, struct kvm_create_guest_memfd *args)
> u64 flags = args->flags;
> u64 valid_flags = 0;
>
> + if (kvm_arch_vm_supports_gmem_shared_mem(kvm))
> + valid_flags |= GUEST_MEMFD_FLAG_SUPPORT_SHARED;
> +
> if (flags & ~valid_flags)
> return -EINVAL;
>
> @@ -501,6 +585,10 @@ 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_vm_supports_gmem_shared_mem(kvm))
> + goto err;
> +
> filemap_invalidate_lock(inode->i_mapping);
>
> start = offset >> PAGE_SHIFT;
> --
> 2.49.0.1045.g170613ef41-goog
>
