On Mon, 31 Aug 2026 at 01:25, Ackerley Tng via B4 Relay <[email protected]> wrote: > > From: Sean Christopherson <[email protected]> > > Stub in global variable to enable in-place guest_memfd private<=>shared > memory conversion, which will eventually be exposed to userspace via a > module param, and wire up the __kvm_is_private_gfn() static call to the > guest_memfd version when in-place conversion is enabled, i.e. when gmem is > the sole authority on private vs. shared memory. > > Cc: Fuad Tabba <[email protected]> > Cc: Xiaoyao Li <[email protected]> > Signed-off-by: Sean Christopherson <[email protected]> > Co-developed-by: Ackerley Tng <[email protected]> > Signed-off-by: Ackerley Tng <[email protected]>
Reviewed-by: Fuad Tabba <[email protected]> Cheers, /fuad > --- > Documentation/virt/kvm/api.rst | 7 +++++++ > include/linux/kvm_host.h | 6 ++++++ > virt/kvm/guest_memfd.c | 26 ++++++++++++++++++++++++++ > virt/kvm/kvm_main.c | 12 +++++++++++- > 4 files changed, 50 insertions(+), 1 deletion(-) > > diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst > index e0430cc750c9e..90a29424c54c8 100644 > --- a/Documentation/virt/kvm/api.rst > +++ b/Documentation/virt/kvm/api.rst > @@ -6382,6 +6382,11 @@ KVM_SET_USER_MEMORY_REGION2, e.g. shared memory can be > lazily mapped/allocated > on-demand. > > When mapping a gfn into the guest, KVM selects shared vs. private, i.e > consumes > +userspace_addr vs. guest_memfd, based on the state in guest_memfd, which is > the > +sole authority on private vs. shared memory. See > :ref:`KVM_CREATE_GUEST_MEMFD` > +to find out more about the creation-time shared/private status. > + > +If in-place conversion is disabled, KVM selects shared vs. private, i.e > consumes > userspace_addr vs. guest_memfd, based on the gfn's > KVM_MEMORY_ATTRIBUTE_PRIVATE > state. At VM creation time, all memory is shared, i.e. the PRIVATE attribute > is '0' for all gfns. Userspace can control whether memory is shared/private > by > @@ -6429,6 +6434,8 @@ the state of a gfn/page as needed. > > The "flags" field is reserved for future extensions and must be '0'. > > +.. _KVM_CREATE_GUEST_MEMFD: > + > 4.142 KVM_CREATE_GUEST_MEMFD > ---------------------------- > > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h > index 3579feb631775..b0110143d2ff8 100644 > --- a/include/linux/kvm_host.h > +++ b/include/linux/kvm_host.h > @@ -2580,6 +2580,8 @@ static inline bool kvm_vm_is_private_gfn(struct kvm > *kvm, gfn_t gfn) > #endif /* CONFIG_KVM_VM_MEMORY_ATTRIBUTES */ > > #ifdef kvm_arch_has_private_mem > +extern bool gmem_in_place_conversion; > + > typedef bool (kvm_is_private_gfn_t)(struct kvm *kvm, gfn_t gfn); > DECLARE_STATIC_CALL(__kvm_is_private_gfn, kvm_is_private_gfn_t); > > @@ -2588,6 +2590,8 @@ static inline bool kvm_is_private_gfn(struct kvm *kvm, > gfn_t gfn) > return static_call(__kvm_is_private_gfn)(kvm, gfn); > } > #else > +#define gmem_in_place_conversion false > + > static inline bool kvm_is_private_gfn(struct kvm *kvm, gfn_t gfn) > { > return false; > @@ -2595,6 +2599,8 @@ static inline bool kvm_is_private_gfn(struct kvm *kvm, > gfn_t gfn) > #endif /* kvm_arch_has_private_mem */ > > #ifdef CONFIG_KVM_GUEST_MEMFD > +bool kvm_gmem_is_private_gfn(struct kvm *kvm, gfn_t gfn); > + > int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot, > gfn_t gfn, kvm_pfn_t *pfn, int *max_order); > #else > diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c > index ce5efcacda36e..cd966eef45b69 100644 > --- a/virt/kvm/guest_memfd.c > +++ b/virt/kvm/guest_memfd.c > @@ -478,6 +478,32 @@ static int kvm_gmem_mmap(struct file *file, struct > vm_area_struct *vma) > return 0; > } > > +bool kvm_gmem_is_private_gfn(struct kvm *kvm, gfn_t gfn) > +{ > + struct kvm_memory_slot *slot = gfn_to_memslot(kvm, gfn); > + struct inode *inode; > + > + if (!slot) > + return 0; > + > + CLASS(gmem_get_file, file)(slot); > + if (!file) > + return 0; > + > + inode = file_inode(file); > + > + /* > + * Rely on the maple tree's internal RCU lock to ensure a stable > result. > + * This result can become stale as soon as the lock is dropped, so the > + * caller _must_ protect consumption of private vs. shared either by > + * holding guest_memfd's invalidate lock for the entire duration, or > by > + * checking mmu_invalidate_retry_gfn() under mmu_lock to serialize > + * against concurrent attribute updates. > + */ > + return kvm_gmem_is_private_mem(inode, kvm_gmem_get_index(slot, gfn)); > +} > +EXPORT_SYMBOL_FOR_KVM_INTERNAL(kvm_gmem_is_private_gfn); > + > static struct file_operations kvm_gmem_fops = { > .mmap = kvm_gmem_mmap, > .open = generic_file_open, > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c > index 78278730b2ae4..46d2e123448c2 100644 > --- a/virt/kvm/kvm_main.c > +++ b/virt/kvm/kvm_main.c > @@ -102,6 +102,10 @@ EXPORT_SYMBOL_FOR_KVM_INTERNAL(halt_poll_ns_shrink); > static bool __ro_after_init allow_unsafe_mappings; > module_param(allow_unsafe_mappings, bool, 0444); > > +#ifdef kvm_arch_has_private_mem > +bool __ro_after_init gmem_in_place_conversion = false; > +#endif > + > /* > * Ordering of locks: > * > @@ -2423,6 +2427,9 @@ static int kvm_vm_ioctl_clear_dirty_log(struct kvm *kvm, > static u64 kvm_supported_vm_mem_attributes(struct kvm *kvm) > { > #ifdef kvm_arch_has_private_mem > + if (gmem_in_place_conversion) > + return 0; > + > if (!kvm || kvm_arch_has_private_mem(kvm)) > return KVM_MEMORY_ATTRIBUTE_PRIVATE; > #endif > @@ -2634,8 +2641,11 @@ EXPORT_STATIC_CALL_GPL(__kvm_is_private_gfn); > > static void kvm_init_memory_attributes(void) > { > + if (gmem_in_place_conversion) > + static_call_update(__kvm_is_private_gfn, > kvm_gmem_is_private_gfn); > #ifdef CONFIG_KVM_VM_MEMORY_ATTRIBUTES > - static_call_update(__kvm_is_private_gfn, kvm_vm_is_private_gfn); > + else > + static_call_update(__kvm_is_private_gfn, > kvm_vm_is_private_gfn); > #endif > } > #else > > -- > 2.55.0.897.gb25b4bd76c-goog > >
