linux-kernel - Re: [PATCH 3/3] KVM: x86: improve reexecute

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Message-ID: <20121128141230.GI928@redhat.com>
Date:	Wed, 28 Nov 2012 16:12:30 +0200
From:	Gleb Natapov <gleb@...hat.com>
To:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
Cc:	Marcelo Tosatti <mtosatti@...hat.com>, Avi Kivity <avi@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>, KVM <kvm@...r.kernel.org>
Subject: Re: [PATCH 3/3] KVM: x86: improve reexecute_instruction

On Tue, Nov 27, 2012 at 11:30:24AM +0800, Xiao Guangrong wrote:
> On 11/27/2012 06:41 AM, Marcelo Tosatti wrote:
> 
> >>
> >> -	return false;
> >> +again:
> >> +	page_fault_count = ACCESS_ONCE(vcpu->kvm->arch.page_fault_count);
> >> +
> >> +	/*
> >> +	 * if emulation was due to access to shadowed page table
> >> +	 * and it failed try to unshadow page and re-enter the
> >> +	 * guest to let CPU execute the instruction.
> >> +	 */
> >> +	kvm_mmu_unprotect_page(vcpu->kvm, gpa_to_gfn(gpa));
> >> +	emulate = vcpu->arch.mmu.page_fault(vcpu, cr3, PFERR_WRITE_MASK, false);
> > 
> > Can you explain what is the objective here?
> > 
> 
> Sure. :)
> 
> The instruction emulation is caused by fault access on cr3. After unprotect
> the target page, we call vcpu->arch.mmu.page_fault to fix the mapping of cr3.
> if it return 1, mmu can not fix the mapping, we should report the error,
> otherwise it is good to return to guest and let it re-execute the instruction
> again.
> 
> page_fault_count is used to avoid the race on other vcpus, since after we
> unprotect the target page, other cpu can enter page fault path and let the
> page be write-protected again.
> 
> This way can help us to detect all the case that mmu can not be fixed.
> 
Can you write this in a comment above vcpu->arch.mmu.page_fault()?

--
			Gleb.
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