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List:       kvm-ia64
Subject:    RE: switching from KVM guest to the host .... TLBs not present?
From:       "Zhang, Xiantao" <xiantao.zhang () intel ! com>
Date:       2009-04-16 9:57:47
Message-ID: 706158FABBBA044BAD4FE898A02E4BC2328721B9 () pdsmsx503 ! ccr ! corp ! intel ! com
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Avi Kivity wrote:
> Zhang, Xiantao wrote:
> > Jes Sorensen wrote:
> > 
> > > Avi Kivity wrote:
> > > 
> > > > Applied, but I note that entering the guest with any lock held is
> > > > problematic, as the guest may spend an arbitrary amount of time in
> > > > guest mode.  Really, entering the guest is almost exactly like
> > > > exiting to userspace. 
> > > > 
> > > Hi Avi,
> > > 
> > > I had a look at this and reworked the locking some, so we don't hold
> > > the slots_lock when entering the guest.
> > > 
> > > How does this look? Xiantao any thoughts of whether it's unsafe to
> > > call kvm_vcpu_post_transition without holding that semaphore? I
> > > believe it should be fine. 
> > > 
> > > I am still seeing issues where the host can get a lock timeout when
> > > running large guests, but the situation seems to be better with
> > > this patch applied. 
> > > 
> > 
> > I remembered this lock should be used to avoid guest memory
> > mappings' changes once vcpu in guest mode before.  Since ia64's
> > memory virtualization locates in VMM, so have to take the lock in
> > VMM, otherwise, guest may access memory with old mapping, but this
> > machanism should be ineffective after optimizing guest's TLB flush
> > in kvm_vcpu_pre_transition.  So this lock shouldn't be held noe.
> > This patch looks fine! Thanks Jes!      
> > 
> > Acked-by : Xiantao Zhang<xiantao.zhang@intel.com>
> > 
> > But anyway, we should implement kvm_arch_flush_shadow to prevent
> > guest using old mapping to access memory, otherwise guest may have
> > issues even probably crash host once guest memory mapping changes
> > frequently.   
> > 
> > 
> 
> Shouldn't we implement kvm_arch_flush_shadow() before we apply this?

It should be another issue.  Even holding the lock in guest mode, it still have \
issue.  Okay, anyway I will work out a patch to fix it.  Xiantao--
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