On Thu, Jan 05, 2023 at 11:23:01AM +0000, Jarkko Sakkinen wrote:

On Fri, Dec 02, 2022 at 02:13:41PM +0800, Chao Peng wrote:

quoted

In memory encryption usage, guest memory may be encrypted with special
key and can be accessed only by the guest itself. We call such memory
private memory. It's valueless and sometimes can cause problem to allow
userspace to access guest private memory. This new KVM memslot extension
allows guest private memory being provided through a restrictedmem
backed file descriptor(fd) and userspace is restricted to access the
bookmarked memory in the fd.

This new extension, indicated by the new flag KVM_MEM_PRIVATE, adds two
additional KVM memslot fields restricted_fd/restricted_offset to allow
userspace to instruct KVM to provide guest memory through restricted_fd.
'guest_phys_addr' is mapped at the restricted_offset of restricted_fd
and the size is 'memory_size'.

The extended memslot can still have the userspace_addr(hva). When use, a
single memslot can maintain both private memory through restricted_fd
and shared memory through userspace_addr. Whether the private or shared
part is visible to guest is maintained by other KVM code.

A restrictedmem_notifier field is also added to the memslot structure to
allow the restricted_fd's backing store to notify KVM the memory change,
KVM then can invalidate its page table entries or handle memory errors.

Together with the change, a new config HAVE_KVM_RESTRICTED_MEM is added
and right now it is selected on X86_64 only.

To make future maintenance easy, internally use a binary compatible
alias struct kvm_user_mem_region to handle both the normal and the
'_ext' variants.

Feels bit hacky IMHO, and more like a completely new feature than
an extension.

Why not just add a new ioctl? The commit message does not address
the most essential design here.

Yes, people can always choose to add a new ioctl for this kind of change
and the balance point here is we want to also avoid 'too many ioctls' if
the functionalities are similar.  The '_ext' variant reuses all the
existing fields in the 'normal' variant and most importantly KVM
internally can reuse most of the code. I certainly can add some words in
the commit message to explain this design choice.

Thanks,
Chao

BR, Jarkko