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

[Avi Kivity <avi@redhat.com>]

On 04/23/2012 10:39 AM, Xiao Guangrong wrote:
> On 04/22/2012 07:39 PM, Avi Kivity wrote:
>
> > 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.
>
> So, IPI is unconditionally sent, do we need introduce a new MODE (say
> STOP_SP_FREE) to reduce IPI?

We can just reuse IN_GUEST_MODE for this (with a comment explaining why).

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