linux-kernel - Re: [PATCH 08/13] KVM: MMU: fask check whether page is writable

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Message-ID: <20120412230828.GA20207@amt.cnet>
Date:	Thu, 12 Apr 2012 20:08:28 -0300
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Xiao Guangrong <xiaoguangrong.eric@...il.com>
Cc:	Avi Kivity <avi@...hat.com>,
	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>,
	LKML <linux-kernel@...r.kernel.org>, KVM <kvm@...r.kernel.org>
Subject: Re: [PATCH 08/13] KVM: MMU: fask check whether page is writable

On Fri, Apr 06, 2012 at 01:54:52AM +0800, Xiao Guangrong wrote:
> Hi Avi,
> 
> Thanks very much for your review!
> 
> Sorry for the delay reply since i was on vacation.
> 
> On 04/01/2012 11:52 PM, Avi Kivity wrote:
> 
> > On 03/29/2012 11:25 AM, Xiao Guangrong wrote:
> >> Using PTE_LIST_WRITE_PROTECT bit in rmap store the write-protect status to
> >> avoid unnecessary shadow page walking
> >>
> >> Also if no shadow page is indirect, the page is write-free
> >>
> >>
> >> @@ -2262,6 +2291,9 @@ static int mmu_need_write_protect(struct kvm_vcpu *vcpu, gfn_t gfn,
> >>  	}
> >>  	if (need_unsync)
> >>  		kvm_unsync_pages(vcpu, gfn);
> >> +
> >> +	*rmap &= ~PTE_LIST_WRITE_PROTECT;
> >> +
> >>
> > 
> > So what are the rules for PTE_LIST_WRITE_PROTECT?  Is is a cache for the
> > mmu_need_write_protect?
> > 
> > I'd like to understand it, I guess it can be set while write protection
> > is unneeded, and cleared on the next check?
> > 
> 
> 
> Yes, it is used as a cache for mmu_need_write_protect.
> 
> When the gfn is protected by sync sp or read-only host page we set this bit,
> and it is be cleared when the sp become unsync or host page becomes writable.

Wouldnt dropping support for shadow entirely make it much simpler? 

The idea to handle RO->RW without mmu_lock is very neat, but the
complexity with shadow is horrible.


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