Re: [Xen-devel] [PATCH] x86/pagewalk: Remove opt_allow_superpage check from guest_can_use_l2_superpages()

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 25/07/17 16:27, Tim Deegan wrote:
> At 16:00 +0100 on 25 Jul (1500998413), Andrew Cooper wrote:
>> The purpose of guest_walk_tables() is to match the behaviour of real 
>> hardware.
>>
>> A PV guest can have 2M superpages in its pagetables, via the M2P and the
>> initial initrd mapping, even if it isn't permitted to create arbitrary 2M
>> superpage mappings.
> Can the domain builder (or Xen?) really give a guest superpage
> mappings for its initrd?  Wouldn't that cause problems for live
> migration?

From Wei's work, dom0 definitely ends up calling put_page_type() on a L2
superpage.

Looking at the dom0 construction code more closely, it appears that Xen
creates the initial p2m in the guest using superpages.

I will s/initrd/initial p2m/ in the commit message.  I don't believe the
domain builder has any way of creating such mappings for a domU, so the
migration aspect is less important.

>
> In any case this patch looks correct: the presence of superpages in PV
> pagetables is decided by the PV MM rules, so the walker should accept them.
>
> Reviewed-by: Tim Deegan <tim@xxxxxxx>

Thanks.

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