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 17:35 +0100, Jan Kiszka wrote:
> On 2011-01-25 15:53, Avi Kivity wrote:
> > On 01/25/2011 04:41 PM, Alex Williamson wrote:
> >> >  >
> >> >  >
> >> >  >  kvm: Allow memory slot array to grow on demand
> >> >  >
> >> >  >  Remove fixed KVM_MEMORY_SLOTS limit, allowing the slot array
> >> >  >  to grow on demand.  Private slots are now allocated at the
> >> >  >  front instead of the end.  Only x86 seems to use private slots,
> >> >
> >> >  Hmm, doesn't current user space expect slots 8..11 to be the private
> >> >  ones and wouldn't it cause troubles if slots 0..3 are suddenly
> >> reserved?
> >>
> >> The private slots aren't currently visible to userspace, they're
> >> actually slots 32..35.  The patch automatically increments user passed
> >> slot ids so userspace has it's own zero-based view of the array.
> >> Frankly, I don't understand why userspace reserves slots 8..11, is this
> >> compatibility with older kernel implementations?
> > 
> > I think so.  I believe these kernel versions are too old now to matter,
> > but of course I can't be sure.
> 
> Will check and enable those 4 additional slots for user space if we no
> longer need to exclude them.

Appears that the history goes something like this...

      * Once upon a time we had 8 memory slots

      * v2.6.24-4949-ge0d62c7 added 4 reserved slots above those 8

      * v2.6.24-4950-gcbc9402 made use of slot 8 for tss

      * v2.6.24-4962-gf78e0e2 made use of slot 9 for tpr

      * v2.6.25-5341-gef2979b bumped the 8 slots to 32

      * v2.6.26-rc1-13-gb7ebfb0 made use of slot 10 for ept

      * v2.6.28-4952-g6fe6397 moved the previously hard coded slots
        8,9,10 up to KVM_MEMORY_SLOTS + {0,1,2}

So we haven't needed to reserve 8..11 for a while and we've never made
use of all 4 private slots.  I should reduce that to 3 in my patch.
Maybe there's a test we could do in userspace to figure out we don't
have to skip those anymore?

Alex