linux-kernel - Re: [RFC PATCH v1 3/5] KVM: Add paravirt kvm_flush_tlb

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Message-ID: <87obq7ge4d.fsf@linux.vnet.ibm.com>
Date:	Wed, 02 May 2012 14:21:14 +0530
From:	Nikunj A Dadhania <nikunj@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>, Avi Kivity <avi@...hat.com>
Cc:	mingo@...e.hu, jeremy@...p.org, mtosatti@...hat.com,
	kvm@...r.kernel.org, x86@...nel.org, vatsa@...ux.vnet.ibm.com,
	linux-kernel@...r.kernel.org, hpa@...or.com
Subject: Re: [RFC PATCH v1 3/5] KVM: Add paravirt kvm_flush_tlb_others

On Tue, 01 May 2012 11:39:36 +0200, Peter Zijlstra <peterz@...radead.org> wrote:
> On Sun, 2012-04-29 at 15:23 +0300, Avi Kivity wrote:
> > On 04/27/2012 07:24 PM, Nikunj A. Dadhania wrote:
> > > flush_tlb_others_ipi depends on lot of statics in tlb.c.  Replicated
> > > the flush_tlb_others_ipi as kvm_flush_tlb_others to further adapt to
> > > paravirtualization.
> > >
> > > Use the vcpu state information inside the kvm_flush_tlb_others to
> > > avoid sending ipi to pre-empted vcpus.
> > >
> > > * Do not send ipi's to offline vcpus and set flush_on_enter flag
> > 
> > get_user_pages_fast() depends on the IPI to hold off page table teardown
> > while they are locklessly walked with interrupts disabled.  If a vcpu
> > were to be preempted while in this critical section, another vcpu
> > tearing down page tables would go ahead and destroy them.  when the
> > preempted vcpu resumes it then touches the freed pages.
> > 
> > We could try to teach kvm and get_user_pages_fast() about this, but this
> > is intrusive.  Another option is to replace the cpu_relax() loop with
> > something that sleeps and is then woken up by the TLB IPI handler if needed.
> 
> I think something like
> 
>   select HAVE_RCU_TABLE_FREE if PARAVIRT
> 
> or somesuch is just about all it takes.
>
[root@...1 linux]# grep HAVE_RCU_TABLE .config
CONFIG_HAVE_RCU_TABLE_FREE=y
[root@...1 linux]# make -j32  -s
mm/memory.c: In function ‘tlb_remove_table_one’:
mm/memory.c:315: error: implicit declaration of function ‘__tlb_remove_table’

I suppose we need to have __tlb_remove_table. Trying to understand what
needs to be done there.

Regards
Nikunj

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