Re: [PATCHv2 0/2] xen/privcmd: prevent page migration for hypercall buffers

[David Vrabel <david.vrabel@citrix.com>]

On 04/08/16 16:16, David Vrabel wrote:
> 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?

Did no one have an opinions on these three options?

David

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