Re: [Xen-devel] Writable page tables questions

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Thu, 2015-01-08 at 12:19 +0100, Tim Deegan wrote:
> At 09:55 +0000 on 06 Jan (1420534536), Ian Campbell wrote:
> > The tlb flushes involved in the above are reasonably expensive, IIRC Xen
> > flip flopped a bit (years ago now) on whether it is worthwhile doing
> > this or not, which is why I'm not sure if it still does or not.
> 
> The current "writable pagetables" code for PV guests emulates the
> write and validates the resulting PTE.  If it passes validation, it
> updates it, without ever making the page actually writable to the
> guest itself.

Indeed, it seems like the mode I was on about was removed 9 years ago:

commit 228f081e08474febb96ee694f6d1b3d6d7465052
Author: kfraser@xxxxxxxxxxxxxxxxxxxxx <kfraser@xxxxxxxxxxxxxxxxxxxxx>
Date:   Fri Aug 11 16:07:22 2006 +0100

    [XEN] Remove batched writable pagetable logic.
    
    Benchmarks show it provides little or no benefit (except
    on synthetic benchmarks). Also it is complicated and
    likely to hinder efforts to reduce lockign granularity.
    
    Signed-off-by: Keir Fraser <keir@xxxxxxxxxxxxx>

$ git describe --contains 228f081e08474febb96ee694f6d1b3d6d7465052
3.0.3-branched~459

So in 3.0.3 apparently.

Ian.


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