Date: Mon, 2 Sep 2013 12:25:18 +0300
From: Gleb Natapov <gleb@redhat.com>
To: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>, linux-kernel@vger.kernel.org,
        kvm@vger.kernel.org, glin@suse.de, agraf@suse.de, brogers@suse.de,
        afaerber@suse.de, lnussel@suse.de, edk2-devel@lists.sf.net,
        stable@vger.kernel.org
Subject: Re: [PATCH] KVM: mmu: allow page tables to be in read-only slots
Message-ID: <20130902092518.GN22899@redhat.com>
References: <1377866497-3866-1-git-send-email-pbonzini@redhat.com>
 <5224584F.6010409@linux.vnet.ibm.com>
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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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