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

[Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>]

On 09/02/2013 05:25 PM, Gleb Natapov wrote:
> 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

I just wonder how to fix this kind of fault. The current patch returns -EACCES
but that will crash the guest. I think we'd better let userspace to fix this
error (let userspace set the D/A bit.)

> first place if ROM page tables have access/dirty bit set and they do.

Yes, so we can not call x86_emulate_instruction() to fix this fault (that function
emulates the access on the first place). Need directly return a MMIO-exit to
userpsace when met this fault? What happen if this fault on pagetable-walking
is trigged in x86_emulate_instruction().?