Re: [Xen-devel] pagetable RO

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 11.10.13 at 09:37, åæ<zhangzhi2022@xxxxxxxxxxx> wrote:

> 
>> Date: Fri, 11 Oct 2013 08:15:43 +0100
>> From: JBeulich@xxxxxxxx 
>> To: zhangzhi2022@xxxxxxxxxxx 
>> CC: xen-devel@xxxxxxxxxxxxxxxxxxxx 
>> Subject: Re: [Xen-devel] pagetable RO
>> 
>> >>> On 11.10.13 at 08:30, åæ<zhangzhi2022@xxxxxxxxxxx> wrote:
>> >       Say, the sub-op: MMU_NORMAL_PT_UPDATE,
>> > if the page that ptr of struct mmu_update points to is PGT_l2_page_table, 
>> > then
>> > the address part of val of struct mmu_update points
>> > to a page, which is PGT_l1_page_table.
>> > However, I couldn't find the protections for this PGT_l1_page_table
>> > page in the do_mmu_update, is it already set as read-only
>> > before launching MMU_UPDATE hypercall?
>> 
> 
>> Of course - a page can't be PGT_l1_page_table when there still is
>> some writable mapping around. 
> Thanks, but when will the page be set as read-only before the MMU_UPDATE 
> hypercall ? is there any hint?

Whenever the guest kernel wants to do that.

>> So for an L2 entry update it is sufficient to verify that all contained L1 
> entries point to R/O
>> pages (and transition them to PGT_l1_page_table if they aren't
>> already).
> 
> Besides, in the function do_mmu_update for an L2 entry update, the new 
> PGT_l1_page_table page, containing all L1 entries,  does not have the process 
> of checking whether they are pointing to the pages of page table type. 

It surely does: mod_l2_entry() -> get_page_from_l2e() ->
get_page_and_type_from_pagenr(mfn, PGT_l1_page_table, ...).

> Instead, R/W flag is removed from the L1 entry only because the page that the 
> entry points to is non-sharable or is a I/O page. What's the reason?

I don't think I understand what you're asking here.

Jan

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