[Xen-devel] [PATCHv2 0/2] xen/privcmd: prevent page migration for hypercall buffers

[David Vrabel <david.vrabel@xxxxxxxxxx>]

Currently libxencall using mlocked buffers for hypercall buffers.
This pages are subject to compaction and page migration. A userspace
process may see a hypercall fail with -EFAULT if a page backing a
hypercall buffer is in the process of being migrated.

Page migration can be prevented by taking an additional reference to
the page.

There are three possible ways to do this:

1. Add a new device which when mmap()'d populated the VMA with
   suitable memory that has an additional reference.  This would give
   VMAs that have appropriate properties set (e.g., VM_DONTCOPY)
   without having to do additional madvise() calls.

2. Add a new ioctl to privcmd to populate a VMA with suitably
   ref-counted pages.  However, mmap() on privcmd is already used for
   foreign mappings and the VMA properties for hypercall buffers and
   foreign mappings might need to be different and the handling of
   unmap will need to be different.  This might be tricky.

3. Add a pair of ioctls to privcmd to lock/unlock a buffer by
   getting/putting an additional reference to the page.  This is
   what's implemented in this series.

Any thoughts on which is the best approach?

Changes in v2:
- Addressed Jan's feedback for #2 (see patch for details).

David


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