Re: Regarding page table management changes from Xen v1to Xen v2 (and v3)

[Anthony Liguori <aliguori@us.ibm.com>]

Keir Fraser wrote:
>
> On 26 Apr 2006, at 19:15, Anthony Liguori wrote:
>
>>> No, these singleton updates on the Linux demand fault path are issued
>>> with the update_va_mapping hypercall. This is possible because the
>>> virtual address that the pte maps is known, so its possible for Xen to
>>> use a linear pagetable to do the update rather than creating an
>>> on-demand mapping to the domain's pagetable page. This is an important
>>> optimization on 32b where Xen address space is tight.
>>>
>>
>> Just to make sure I understand, we do the linear mapping for 
>> performance (so we don't have to keep remapping) but not necessarily 
>> to save VA space since the linear mapping uses a fixed amount of space.
>>
>> Is there any reason that we would strictly require a linear mapping 
>> (other than it being convenient)?
>
> On 64b we could use the 1:1 mapping of all physmem that Xen has. 
> Obviously that is infeasible on 32b. In fact the interface (passing a 
> virtual address) pretty much means we have to use a linear mapping in 
> both cases.

Couldn't we temporarily map the page table page?  Not that we would 
actually want to do that.  Just trying to confirm my understand.

Regards,

Anthony Liguori

>  -- Keir
>