linux-kernel - Re: [PATCH] KVM: MMU: avoid fast page fault fixing mmio page fault

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Message-ID: <51E784CB.1040402@linux.vnet.ibm.com>
Date:	Thu, 18 Jul 2013 14:01:47 +0800
From:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
To:	Gleb Natapov <gleb@...hat.com>
CC:	markus@...ppelsdorf.de, mtosatti@...hat.com, pbonzini@...hat.com,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH] KVM: MMU: avoid fast page fault fixing mmio page fault

On 07/18/2013 01:31 PM, Gleb Natapov wrote:
> On Thu, Jul 18, 2013 at 12:52:37PM +0800, Xiao Guangrong wrote:
>> Currently, fast page fault tries to fix mmio page fault when the
>> generation number is invalid (spte.gen != kvm.gen) and returns to
>> guest to retry the fault since it sees the last spte is nonpresent
>> which causes infinity loop
>>
>> It can be triggered only on AMD host since the mmio page fault is
>> recognized as ept-misconfig
>>
> We still call into regular page fault handler from ept-misconfig
> handler, but fake zero error_code we provide makes page_fault_can_be_fast()
> return false.

Yes.

> 
> Shouldn't shadow paging trigger this too? I haven't encountered this on
> Intel without ept.

Since currently fast page fault only works for direct mmu. :)

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