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

Gleb Natapov
2013-09-02T10:07:47Z

On Mon, Sep 02, 2013 at 06:00:39PM +0800, Xiao Guangrong wrote:
> 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.)
>
Ugh, this is not good. Missed that. Don't know what real HW will do
here, but the easy thing for us to do would be to just return success.

> > 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().?
I think we should not return MMIO-exit to userspace. Either ignore write attempt
or kill a guest.

--
Gleb.