Re: [Xen-devel] why xen use x86_emulation() in page_fault

[cc Luit <universalbillow@xxxxxxxxx>]

On Tue, Oct 11, 2011 at 8:21 PM, Tim Deegan <tim@xxxxxxx> wrote:

At 20:03 +0800 on 11 Oct (1318363429), cc Luit wrote:
> I've read a slide said that in Lazy mode:
>  when guest os modify the GPT, do not emulate (there is no
> write-protected PTE, so guest can directly modify it)
>  1) when the access right ascension, the guest OS will INVLPG to shootdown
> TLB, so hypervisor can catch the INPLPG inst to sync  up the SPT/GPT
>    2) when access right down, when guest OS access this page it will trap to
> xen, xen will catch #PF to sync up SPT/GPT,
>
> I'm not sure if this can work?

> I don't quite understand your English, but yes, you can let the guest
> write to page tables and sync up the shadows afterwards.  You need to
> sync the shadows whenever real hardware would re-read pagetables.  That
> is, at least:
>  - When the guest executes INVLPG
>  - When the guest writes to CR3
>  - When the shadowing code adds a new l2e, l3e or l4e that points to an
>   existing shadow.
> (and possibly some other cases I don;t remember right now).

really sorry about my poor english:-) I will consider your suggestions carefully~ thanks!

> > Why do you want to avoid calling the emulator? What is your overall goal?
> > It might be that tinkering in the shadow pagetables isn't the best way
> > to acheive it.
>
> because we're doing some research of security aspect about xen, what's our
> goal is avoid xen to access the HVM's memory in the page fault situation,
> it's hard to say it out in short words, we have thought a lot of ways but
> there is no a simpler one than avoiding the emulation in page_fault.

> Ah, I see - this is following on from the CloudVisor work, is it?

yeah~ CloudVisor was just our lab's work, but ours is a little different:-)

> Why don't you just use EPT/NPT instead of shadow pagetables?  That code
> is a lot simpler and the end result is easier to validate than shadow
> pagetables are.  Is it particularly important to support shadow pagetables?

yeah, we have consider that, and of course the EPT is much more convenient, BTW, do you know is there any simulator that can well support EPT of xen? the shadow page's problem is a little relevent to the selection of simulator~

> In either case, there are some instructions that Xen will always
> emulate - for example:
>  - MMIO accesses to emulated hardware
>  - Some real-mode instructions on older Intel hardware

yeah, that's true, and we are also considering it:-)

> so even if you rewrote the shadow pagetable code not to emulate PT writes
> you still have to handle those.

Cheers,

Tim.

-- 
- Luit @ Parallel Processing Institute, Fudan University 
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