Re: 2MB page PV guest support clarification

[Jeremy Fitzhardinge <jeremy@goop.org>]

Mick Jordan wrote:
> I was inspired by the talk from Ben Serebrin at this weeks' summit to 
> investigate using 2MB pages in my Java VM on Xen for x86-64.
>
> I think I have done everything correctly, but Xen (3.1.4) rejects my 
> attempt to set the PSE bit in the L2 frame for the 2MB page. Looking 
> at the Xen code the L2_DISALLOW_MASK (0xFF800180U) simply rejects the 
> update if the PSE bit is set.

Yes.  Xen doesn't support large mappings for PV guests.  However, 
there's a lot less to worry about for PV guests compared to hvm guests. 
A PV guest directly uses the CPU's pagetable+tlb hardware, and so a tlb 
miss results in a single simple walk of the pagetable, and the overall 
tlb pressure is a lot less.  The desire to use large pages for hvm 
guests is driven by the cost of a tlb miss when you have 4k guest pages 
layered on 4k host pages, resulting in 24 memory accesses in the worst 
case; a PV tlb miss is no more expensive than a native tlb miss by 
comparison.

Large pages could potentially reduce the cost of a PV tlb miss as well, 
but also pose quite a few tradeoffs.  You can't generally use large 
mappings for the kernel, as you can native, because of all the pages 
which need RO mappings (pagetables, gdt, etc).  Also, IO and the balloon 
driver operate at 4k page resolution, so breaking a contiguous 2M page 
would require the mapping to be shattered.

> I found some posts from quite a while ago on xen-devel discussing 
> patches to allow large pages, so my question is for clarification  on 
> the status of this feature. I.e., is it in any stable release and if 
> so what version?

Its a work in progress, but there's nothing usable yet, as far as I know.

    J