Re: [PATCH 0/2] Simplify RCU freeing of shadow pages

[Avi Kivity <avi@redhat.com>]

On 04/21/2012 05:31 AM, Marcelo Tosatti wrote:
> On Thu, Apr 19, 2012 at 07:26:35PM +0300, Avi Kivity wrote:
> > This patchset simplifies the freeing by RCU of mmu pages.
> > 
> > Xiao, I'm sure you thought of always freeing by RCU.  Why didn't you choose
> > this way?  I saves a couple of atomics in the fast path.
> > 
> > Avi Kivity (2):
> >   KVM: MMU: Always free shadow pages using RCU
> >   KVM: MMU: Recover space used by rcu_head in struct kvm_mmu_page
> > 
> >  arch/x86/include/asm/kvm_host.h |    9 +++---
> >  arch/x86/kvm/mmu.c              |   58 ++++++++-------------------------------
> >  2 files changed, 15 insertions(+), 52 deletions(-)
>
> Check Documentation/RCU/checklist.txt item 8.
>
>         a.      Keeping a count of the number of data-structure elements
>                 used by the RCU-protected data structure, including
>                 those waiting for a grace period to elapse.  Enforce a
>                 limit on this number, stalling updates as needed to allow
>                 previously deferred frees to complete.  Alternatively,
>                 limit only the number awaiting deferred free rather than
>                 the total number of elements.
>

That's true before and after the patch.  Currently the amount of memory
that depends on rcu for freeing is unbounded.

Maybe we should protect the fast path using local_irq_disable() instead
of rcu_read_lock(), like x86 page tables.  That means that
kvm_flush_remote_tlbs() needs to ignore OUTSIDE_GUEST_MODE when IPIing
vcpu threads.

-- 
error compiling committee.c: too many arguments to function