Re: [Xen-devel] why xen use x86_emulat ion() in page_fault

[cc Luit <universalbillow@xxxxxxxxx>]

Sorry about that:-)

Yeah, I've seen this comments, I understand what it says in the before part, but not the last sentence, what does it mean by "non-user write"? and what's more, in the code following is: 
    if ( is_hvm_domain(d)
         && unlikely(!hvm_wp_enabled(v))
         && regs->error_code == (PFEC_write_access|PFEC_page_present)
         && mfn_valid(gmfn) )
    {
        perfc_incr(shadow_fault_emulate_wp);
        goto emulate;
    }

there is not opinion show that the guest is to write through read-only PTEs, but just a hvm_wp_enabled but mfn_valid address, does it because only the PTE is hvm_wp_enabled?
 
another question is that if for some reasons I want to design that the Guest PTE is not read-only, which means in the page_fault situation I don't want xen to emulate, is there any functionability or feasibility problems?
 
thanks for your answer:)

On Tue, Oct 11, 2011 at 5:27 PM, George Dunlap <George.Dunlap@xxxxxxxxxxxxx> wrote:

The comments in xen/arch/mm/shadow/multi.c:sh_page_fault() about emulation say:
   /* Need to emulate accesses to page tables */
and
   /* In HVM guests, we force CR0.WP always to be set, so that the
    * pagetables are always write-protected.  If the guest thinks
    * CR0.WP is clear, we must emulate faulting supervisor writes to
    * allow the guest to write through read-only PTEs.  Emulate if the
    * fault was a non-user write to a present page.  */

Do you have some question about these?

Regarding NPT/EPT, as far as I know, the only reason to do emulation
is when the guest is doing MMIO.  The whole point of MMIO is to
emulate access to devices.

 -George

On Tue, Oct 11, 2011 at 2:58 AM, cc Luit <universalbillow@xxxxxxxxx> wrote:
> Hi, everyone, I have a question,
> in the shadow_page_fault or ept mechanism, xen will use the x86_emulation
> for some instructions, I'm wondering why it must use it, if after we fix the
> SPT or EPT table, just VMEntry to HVM to re-excute this instruction  but not
> emulate in xen, is there some problems?
>

> can sb. tell me why?
> thanks:)
> --
> - Luit @ Parallel Processing Institute, Fudan University
>

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-- 
- Luit @ Parallel Processing Institute, Fudan University 
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