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

Gleb Natapov
2013-09-02T16:16:17Z

On Mon, Sep 02, 2013 at 06:00:08PM +0200, Paolo Bonzini wrote:
> 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).
>
They do it for a good reason, dirty bit exists only on the bottom level :)

--
Gleb.