On 05/03/2012 04:23 PM, Xiao Guangrong wrote:
> On 05/03/2012 07:22 PM, Avi Kivity wrote:
>
> > Currently we flush the TLB while holding mmu_lock. This
> > increases the lock hold time by the IPI round-trip time, increasing
> > contention, and makes dropping the lock (for latency reasons) harder.
> >
> > This patch changes TLB management to be usable locklessly, introducing
> > the following APIs:
> >
> > kvm_mark_tlb_dirty() - mark the TLB as containing stale entries
> > kvm_cond_flush_remote_tlbs() - flush the TLB if it was marked as
> > dirty
> >
> > These APIs can be used without holding mmu_lock (though if the TLB
> > became stale due to shadow page table modifications, typically it
> > will need to be called with the lock held to prevent other threads
> > from seeing the modified page tables with the TLB unmarked and unflushed)/
> >
> > Signed-off-by: Avi Kivity <avi@xxxxxxxxxx>
> > ---
> > Documentation/virtual/kvm/locking.txt | 14 ++++++++++++++
> > arch/x86/kvm/paging_tmpl.h | 4 ++--
> > include/linux/kvm_host.h | 22 +++++++++++++++++++++-
> > virt/kvm/kvm_main.c | 29 ++++++++++++++++++++---------
> > 4 files changed, 57 insertions(+), 12 deletions(-)
> >
> > diff --git a/Documentation/virtual/kvm/locking.txt b/Documentation/virtual/kvm/locking.txt
> > index 3b4cd3b..f6c90479 100644
> > --- a/Documentation/virtual/kvm/locking.txt
> > +++ b/Documentation/virtual/kvm/locking.txt
> > @@ -23,3 +23,17 @@ Arch: x86
> > Protects: - kvm_arch::{last_tsc_write,last_tsc_nsec,last_tsc_offset}
> > - tsc offset in vmcb
> > Comment: 'raw' because updating the tsc offsets must not be preempted.
> > +
> > +3. TLB control
> > +--------------
> > +
> > +The following APIs should be used for TLB control:
> > +
> > + - kvm_mark_tlb_dirty() - indicates that the TLB is out of sync wrt
> > + either guest or host page tables.
> > + - kvm_flush_remote_tlbs() - unconditionally flush the tlbs
> > + - kvm_cond_flush_remote_tlbs() - flush the TLBs if previously marked
> > +
> > +These may be used without mmu_lock, though kvm_mark_tlb_dirty() needs to be
> > +used while holding mmu_lock if it is called due to host page table changes
> > +(contrast to guest page table changes).
>
>
> In these patches, it seems that kvm_mark_tlb_dirty is always used
> under the protection of mmu-lock, yes?
Correct. It's possible we'll find a use outside mmu_lock, but this
isn't likely.
> If both kvm_mark_tlb_dirty and kvm_cond_flush_remote_tlbs are use
> out of mmu-lock, i think we can use kvm_flush_remote_tlbs instead.
>
> If it is so, dirtied_count/flushed_count need not be atomic.
But we always mark with mmu_lock held.
>
> And, it seems there is a bug:
>
> VCPU 0 VCPU 1
>
> hold mmu-lock
> zap spte which points to 'gfn'
> mark_tlb_dirty
> release mmu-lock
> hold mmu-lock
> rmap_write-protect:
> see no spte pointing to gfn
> tlb did not be flushed
> release mmu-lock
>
> write gfn by guest
> OOPS!!!
>
> kvm_cond_flush_remote_tlbs
>
> We need call kvm_cond_flush_remote_tlbs in rmap_write-protect
> unconditionally?
Yes, that's the point. We change
spin_lock(mmu_lock)
conditionally flush the tlb
spin_unlock(mmu_lock)
to
spin_lock(mmu_lock)
conditionally mark the tlb as dirty
spin_unlock(mmu_lock)
kvm_cond_flush_remote_tlbs()
but that means the entire codebase has to be converted.
--
error compiling committee.c: too many arguments to function
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