Re: [RFC PATCH 0/2] Expose available KVM free memory slot count to help avoid aborts

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Jan 21, 2011 at 04:48:02PM -0700, Alex Williamson wrote:
> When doing device assignment, we use cpu_register_physical_memory() to
> directly map the qemu mmap of the device resource into the address
> space of the guest.  The unadvertised feature of the register physical
> memory code path on kvm, at least for this type of mapping, is that it
> needs to allocate an index from a small, fixed array of memory slots.
> Even better, if it can't get an index, the code aborts deep in the
> kvm specific bits, preventing the caller from having a chance to
> recover.
> 
> It's really easy to hit this by hot adding too many assigned devices
> to a guest (pretty easy to hit with too many devices at instantiation
> time too, but the abort is slightly more bearable there).
> 
> I'm assuming it's pretty difficult to make the memory slot array
> dynamically sized.  If that's not the case, please let me know as
> that would be a much better solution.
> 
> I'm not terribly happy with the solution in this series, it doesn't
> provide any guarantees whether a cpu_register_physical_memory() will
> succeed, only slightly better educated guesses.
> 
> Are there better ideas how we could solve this?  Thanks,
> 
> Alex

Put the table in qemu memory, make kvm access it with copy from/to user?
It can then be any size ...

> ---
> 
> Alex Williamson (2):
>       device-assignment: Count required kvm memory slots
>       kvm: Allow querying free slots
> 
> 
>  hw/device-assignment.c |   59 +++++++++++++++++++++++++++++++++++++++++++++++-
>  hw/device-assignment.h |    3 ++
>  kvm-all.c              |   16 +++++++++++++
>  kvm.h                  |    2 ++
>  4 files changed, 79 insertions(+), 1 deletions(-)