Fuad Tabba <tabba@xxxxxxxxxx> writes: > Add support for mmap() and fault() for guest_memfd backed memory > in the host for VMs that support in-place conversion between > shared and private. To that end, this patch adds the ability to > check whether the VM type supports in-place conversion, and only > allows mapping its memory if that's the case. > > This patch introduces the configuration option KVM_GMEM_SHARED_MEM, > which enables support for in-place shared memory. > > It also introduces the KVM capability KVM_CAP_GMEM_SHARED_MEM, which > indicates that the host can create VMs that support shared memory. > Supporting shared memory implies that memory can be mapped when shared > with the host. > > Signed-off-by: Fuad Tabba <tabba@xxxxxxxxxx> > --- > include/linux/kvm_host.h | 15 ++++++- > include/uapi/linux/kvm.h | 1 + > virt/kvm/Kconfig | 5 +++ > virt/kvm/guest_memfd.c | 92 ++++++++++++++++++++++++++++++++++++++++ > virt/kvm/kvm_main.c | 4 ++ > 5 files changed, 116 insertions(+), 1 deletion(-) > > <snip> At the guest_memfd call on 2025-05-01, we discussed that if guest_memfd is created with GUEST_MEMFD_FLAG_SUPPORT_SHARED set, then if slot->userspace_addr != 0, we would validate that the folio slot->userspace_addr points to matches up with the folio guest_memfd would return for the same offset. I can think of one way to do this validation, which is to call KVM's hva_to_pfn() function and then call kvm_gmem_get_folio() on the fd and offset, and then check that the PFNs are equal. However, that would cause the page to be allocated. Any ideas on how we could do this validation without allocating the page?
