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

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

Ian Pratt wrote:
>> In another example, mmapped access to a file, the new 
>> approach will incur these two extra faults for every page 
>> fault required to bring a file page to page cache, right?
>>     
>
> 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)?

Thanks,

Anthony Liguori

> Ian
>
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