From d2042d8f96ddefdeee823737f813efe3ab4b4e8d Mon Sep 17 00:00:00 2001 From: Sean Christopherson Date: Fri, 3 Oct 2025 16:25:54 -0700 Subject: KVM: Rework KVM_CAP_GUEST_MEMFD_MMAP into KVM_CAP_GUEST_MEMFD_FLAGS Rework the not-yet-released KVM_CAP_GUEST_MEMFD_MMAP into a more generic KVM_CAP_GUEST_MEMFD_FLAGS capability so that adding new flags doesn't require a new capability, and so that developers aren't tempted to bundle multiple flags into a single capability. Note, kvm_vm_ioctl_check_extension_generic() can only return a 32-bit value, but that limitation can be easily circumvented by adding e.g. KVM_CAP_GUEST_MEMFD_FLAGS2 in the unlikely event guest_memfd supports more than 32 flags. Reviewed-by: Ackerley Tng Tested-by: Ackerley Tng Reviewed-by: David Hildenbrand Link: https://lore.kernel.org/r/20251003232606.4070510-2-seanjc@google.com Signed-off-by: Sean Christopherson --- Documentation/virt/kvm/api.rst | 10 +++++++--- 1 file changed, 7 insertions(+), 3 deletions(-) (limited to 'Documentation') diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst index 6ae24c5ca559..7ba92f2ced38 100644 --- a/Documentation/virt/kvm/api.rst +++ b/Documentation/virt/kvm/api.rst @@ -6432,9 +6432,13 @@ 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_GUEST_MEMFD_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. +The capability KVM_CAP_GUEST_MEMFD_FLAGS enumerates the `flags` that can be +specified via KVM_CREATE_GUEST_MEMFD. Currently defined flags: + + ============================ ================================================ + GUEST_MEMFD_FLAG_MMAP Enable using mmap() on the guest_memfd file + descriptor. + ============================ ================================================ 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 -- cgit v1.2.3 From fe2bf6234e947bf5544db6d386af1df2a8db80f3 Mon Sep 17 00:00:00 2001 From: Sean Christopherson Date: Fri, 3 Oct 2025 16:25:55 -0700 Subject: KVM: guest_memfd: Add INIT_SHARED flag, reject user page faults if not set Add a guest_memfd flag to allow userspace to state that the underlying memory should be configured to be initialized as shared, and reject user page faults if the guest_memfd instance's memory isn't shared. Because KVM doesn't yet support in-place private<=>shared conversions, all guest_memfd memory effectively follows the initial state. Alternatively, KVM could deduce the initial state based on MMAP, which for all intents and purposes is what KVM currently does. However, implicitly deriving the default state based on MMAP will result in a messy ABI when support for in-place conversions is added. For x86 CoCo VMs, which don't yet support MMAP, memory is currently private by default (otherwise the memory would be unusable). If MMAP implies memory is shared by default, then the default state for CoCo VMs will vary based on MMAP, and from userspace's perspective, will change when in-place conversion support is added. I.e. to maintain guest<=>host ABI, userspace would need to immediately convert all memory from shared=>private, which is both ugly and inefficient. The inefficiency could be avoided by adding a flag to state that memory is _private_ by default, irrespective of MMAP, but that would lead to an equally messy and hard to document ABI. Bite the bullet and immediately add a flag to control the default state so that the effective behavior is explicit and straightforward. Fixes: 3d3a04fad25a ("KVM: Allow and advertise support for host mmap() on guest_memfd files") Cc: David Hildenbrand Reviewed-by: Fuad Tabba Tested-by: Fuad Tabba Reviewed-by: Ackerley Tng Tested-by: Ackerley Tng Reviewed-by: David Hildenbrand Link: https://lore.kernel.org/r/20251003232606.4070510-3-seanjc@google.com Signed-off-by: Sean Christopherson --- Documentation/virt/kvm/api.rst | 5 +++++ 1 file changed, 5 insertions(+) (limited to 'Documentation') diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst index 7ba92f2ced38..754b662a453c 100644 --- a/Documentation/virt/kvm/api.rst +++ b/Documentation/virt/kvm/api.rst @@ -6438,6 +6438,11 @@ specified via KVM_CREATE_GUEST_MEMFD. Currently defined flags: ============================ ================================================ GUEST_MEMFD_FLAG_MMAP Enable using mmap() on the guest_memfd file descriptor. + GUEST_MEMFD_FLAG_INIT_SHARED Make all memory in the file shared during + KVM_CREATE_GUEST_MEMFD (memory files created + without INIT_SHARED will be marked private). + Shared memory can be faulted into host userspace + page tables. Private memory cannot. ============================ ================================================ When the KVM MMU performs a PFN lookup to service a guest fault and the backing -- cgit v1.2.3