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

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Message-ID: <20121128215750.GA10039@amt.cnet>
Date:	Wed, 28 Nov 2012 19:57:50 -0200
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
Cc:	Gleb Natapov <gleb@...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 Wed, Nov 28, 2012 at 10:59:35PM +0800, Xiao Guangrong wrote:
> On 11/28/2012 10:12 PM, Gleb Natapov wrote:
> > 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()?
> 
> Okay, if Marcelo does not object this way. :)

I do object, since it is possible to detect precisely the condition by 
storing which gfns have been cached.

Then, Xiao, you need a way to handle large read-only sptes.

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