linux-kernel - Re: [PATCH] KVM: mmu: allow page tables to be in read-only slots

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Date:	Mon, 2 Sep 2013 12:25:18 +0300
From:	Gleb Natapov <gleb@...hat.com>
To:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
Cc:	Paolo Bonzini <pbonzini@...hat.com>, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org, glin@...e.de, agraf@...e.de, brogers@...e.de,
	afaerber@...e.de, lnussel@...e.de, edk2-devel@...ts.sf.net,
	stable@...r.kernel.org
Subject: Re: [PATCH] KVM: mmu: allow page tables to be in read-only slots

On Mon, Sep 02, 2013 at 05:20:15PM +0800, Xiao Guangrong wrote:
> On 08/30/2013 08:41 PM, Paolo Bonzini wrote:
> > Page tables in a read-only memory slot will currently cause a triple
> > fault because the page walker uses gfn_to_hva and it fails on such a slot.
> > 
> > OVMF uses such a page table; however, real hardware seems to be fine with
> > that as long as the accessed/dirty bits are set.  Save whether the slot
> > is readonly, and later check it when updating the accessed and dirty bits.
> 
> Paolo, do you know why OVMF is using readonly memory like this?
> 
Just a guess, but perhaps they want to move to paging mode as early as
possible even before memory controller is fully initialized.

> AFAIK, The fault trigged by this kind of access can hardly be fixed by
> userspace since the fault is trigged by pagetable walking not by the current
> instruction. Do you have any idea to let uerspace emulate it properly?
Not sure what userspace you mean here, but there shouldn't be a fault in the
first place if ROM page tables have access/dirty bit set and they do.

--
			Gleb.
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