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

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Date:	Tue, 23 Aug 2011 04:21:05 +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 03/11] KVM: x86: retry non-page-table writing instruction

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.
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