linux-kernel - Re: [PATCH 03/11] KVM: x86: retry non-page-table writing instruction

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Message-ID: <20110822204209.GA3490@amt.cnet>
Date:	Mon, 22 Aug 2011 17:42:10 -0300
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Xiao Guangrong <xiaoguangrong@...fujitsu.com>
Cc:	Avi Kivity <avi@...hat.com>, LKML <linux-kernel@...r.kernel.org>,
	KVM <kvm@...r.kernel.org>
Subject: Re: [PATCH 03/11] KVM: x86: retry non-page-table writing instruction

On Tue, Aug 23, 2011 at 04:21:05AM +0800, Xiao Guangrong wrote:
> On 08/23/2011 03:59 AM, Marcelo Tosatti wrote:
> 
> >> +	if (!vcpu->arch.mmu.direct_map && !mmu_is_nested(vcpu))
> >> +		gpa = kvm_mmu_gva_to_gpa_write(vcpu, cr2, NULL);
> > 
> > Why write? 
> > 
> 
> Since the fault is caused by page table written, and the 'gpa' can
> be written after instruction is retied.
> 
> >> +	kvm_mmu_unprotect_page(vcpu->kvm, gpa >> PAGE_SHIFT);
> >> +
> >> +	/*
> >> +	 * The shadow pages have been zapped, then we call the page
> >> +	 * fault path to change the mapping to writable.
> >> +	 */
> >> +	vcpu->arch.mmu.page_fault(vcpu, cr2, PFERR_WRITE_MASK, true);
> > 
> > I don't see why is this necessary. Just allowing the instruction to
> > proceed should be enough?
> > 
> 
> It used to avoid later VM-exit, since we will retry the instruction
> but the mapped is still read-only. So we can it to let the mapping become
> writable to avoid page fault again.

Its not like this case is performance sensitive. Usually optimizing
things without the need for it leads to bad results. So please drop 
this.

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