Re: [PATCH v9 07/17] KVM: guest_memfd: Allow host to map guest_memfd() pages

[Fuad Tabba <tabba@google.com>]

Hi Patrick,

On Wed, 14 May 2025 at 11:07, Roy, Patrick <roypat@amazon.co.uk> wrote:
>
> 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@google.com>
> > Signed-off-by: Ackerley Tng <ackerleytng@google.com>
> > Signed-off-by: Fuad Tabba <tabba@google.com>
> > ---
> >  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

Yes we did. I missed it in this patch. I'll fix it.

> 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 :/

Not quite, but I'll sort it out.

Thanks,
/fuad

> 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
> >
>