linux-kernel - Re: [PATCH 02/15] KVM: MMU: do not update slot bitmap if spte is nonpresent

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Date:	Tue, 21 Jun 2011 02:32:54 +0800
From:	Xiao Guangrong <xiaoguangrong@...fujitsu.com>
To:	Marcelo Tosatti <mtosatti@...hat.com>
CC:	Avi Kivity <avi@...hat.com>, LKML <linux-kernel@...r.kernel.org>,
	KVM <kvm@...r.kernel.org>
Subject: Re: [PATCH 02/15] KVM: MMU: do not update slot bitmap if spte is
 nonpresent

On 06/21/2011 12:28 AM, Marcelo Tosatti wrote:
> On Tue, Jun 07, 2011 at 08:59:25PM +0800, Xiao Guangrong wrote:
>> Set slot bitmap only if the spte is present
>>
>> Signed-off-by: Xiao Guangrong <xiaoguangrong@...fujitsu.com>
>> ---
>>  arch/x86/kvm/mmu.c |   15 +++++++--------
>>  1 files changed, 7 insertions(+), 8 deletions(-)
>>
>> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
>> index cda666a..125f78d 100644
>> --- a/arch/x86/kvm/mmu.c
>> +++ b/arch/x86/kvm/mmu.c
>> @@ -743,9 +743,6 @@ static int rmap_add(struct kvm_vcpu *vcpu, u64 *spte, gfn_t gfn)
>>  	struct kvm_mmu_page *sp;
>>  	unsigned long *rmapp;
>>  
>> -	if (!is_rmap_spte(*spte))
>> -		return 0;
>> -
> 
> Not sure if this is safe, what if the spte is set as nonpresent but
> rmap not removed?

It can not happen, since when we set the spte as nonpresent, we always use
drop_spte to remove the rmap, we also do it in set_spte()

> 
> BTW i don't see what patch 1 and this have to do with the goal
> of the series.
> 
>

There are the preparing work for mmio page fault:
- Patch 1 fix the bug in walking shadow page, so we can safely use it to
  lockless-ly walk shadow page
- Patch 2 can avoid add rmap for the mmio spte :-)

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