Re: [PATCH 1/4] KVM: Add APIs for unlocked TLB flush

[Avi Kivity <avi@redhat.com>]

On 05/07/2012 10:06 AM, Xiao Guangrong wrote:
> On 05/03/2012 10:11 PM, Avi Kivity wrote:
>
> > 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@redhat.com>
> >>> ---
> >>>  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 we need call kvm_mark_tlb_dirty outside mmu-lock, just use
> kvm_flush_remote_tlbs instead:
>
> if (need-flush-tlb)
> 	flush = true;
>
> do something...
>
> if (flush)
> 	kvm_flush_remote_tlbs

It depends on how need-flush-tlb is computed.  If it depends on sptes,
then we mush use kvm_cond_flush_remote_tlbs().

> > 
> >> 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.
> > 
>
>
> Yes, so, we can change kvm_mark_tlb_dirty to:
>
> +static inline void kvm_mark_tlb_dirty(struct kvm *kvm)
> +{
> +	/*
> +	 * Make any changes to the page tables visible to remote flushers.
> +	 */
> +	smb_mb();
> +	kvm->tlb_state.dirtied_count++;
> +}
>

Yes.  We'll have to change it again if we ever dirty sptes outside the
lock, but that's okay.

-- 
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