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

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Message-ID: <20130718060628.GA15613@redhat.com>
Date:	Thu, 18 Jul 2013 09:06:28 +0300
From:	Gleb Natapov <gleb@...hat.com>
To:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.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 Thu, Jul 18, 2013 at 02:01:47PM +0800, Xiao Guangrong wrote:
> 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. :)
Ah, yes. So with shadow page and paging disabled in a guest is can
happen eventually, but we do not trigger it for some reason?

--
			Gleb.
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