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

[Marcelo Tosatti <mtosatti@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.

Its not difficult to either increase the maximum number (defined as
32 now in both qemu and kernel) of static slots, or support dynamic
increases, if it turns out to be a performance issue.

But you'd probably want to fix the abort for currently supported kernels
anyway.

> 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,

Why can't cpu_register_physical_memory() return an error so you can
fallback to slow mode or cancel device insertion?