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

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Date:	Mon, 02 Sep 2013 18:00:08 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Gleb Natapov <gleb@...hat.com>
CC:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.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

Il 02/09/2013 11:25, Gleb Natapov ha scritto:
> 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.

More precisely they want to move to 64-bit mode as early as possible,
and that requires paging.

>> 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.

Yep.  Actually they do not set dirty on the PD/PDP/PML4, only on the
bottom level (which is also fine).

Paolo

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