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

[Alex Williamson <alex.williamson@redhat.com>]

On Tue, 2011-01-25 at 12:23 +0200, Avi Kivity wrote:
> On 01/25/2011 07:37 AM, Alex Williamson wrote:
> > On Mon, 2011-01-24 at 08:44 -0700, Alex Williamson wrote:
> > >  I'll look at how we might be
> > >  able to allocate slots on demand.  Thanks,
> >
> > Here's a first cut just to see if this looks agreeable.  This allows the
> > slot array to grow on demand.  This works with current userspace, as
> > well as userspace trivially modified to double KVMState.slots and
> > hotplugging enough pci-assign devices to exceed the previous limit (w/&
> > w/o ept).  Hopefully I got the rcu bits correct.  Does this look like
> > the right path?
> 
> This can be trivially exhausted to pin all RAM.

What's a reasonable upper limit?  A PCI device can have at most 6 MMIO
BARs, each taking one slot.  It might also support MSI-X in a way that
splits a BAR, so an absolute max of 7 slots per PCI device.  Assuming
only 1 PCI bus for the moment and also assuming assigned devices are
typically single function, 32 devices * 7 slots/device = 224 slots, so
maybe a 256 limit?  Maybe we even bump it up to a limit of 64 devices
with a slot limit of 512.  It would be easier in device assignment code
to keep track of a number of devices limit than trying to guess whether
slots will be available when we need them.  Thanks,

Alex