Xen devel - Apr 2006 - RE: Regarding page table management changes from Xen v1to Xen v2 (and v3)

> 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.

Ian

_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xensource.com
http://lists.xensource.com/xen-devel

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.

  -- Keir


_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xensource.com
http://lists.xensource.com/xen-devel

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
>
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@lists.xensource.com
> http://lists.xensource.com/xen-devel
>   

_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xensource.com
http://lists.xensource.com/xen-devel

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
>

_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xensource.com
http://lists.xensource.com/xen-devel