Re: [Xen-devel] Re: [Xen-users] how to disable batching of page table entries updates for guest OS

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Tue, 2011-05-10 at 03:27 +0100, Joseph Glanville wrote:
> Hi,
> 
> This is a much better question for the xen-devel list.
> I am cc'ing the list in.
> 
> Sounds like an intersting experiment however, goodluck.
> 
> Joseph.
> 
> On 9 May 2011 12:27, Qiang Zeng <zengqiang.buaa@xxxxxxxxx> wrote:
> > Xen has a perf. enhancement by batching page table updates for the guest OS.
> > I'm trying to measure the impact on the guest OS's performance, when this
> > enhancement is disabled.
> >
> > But I don't know how to disable it. I tried modifying do_mmu_update in
> > xen/arch/x86/mm.c, which is to process the batch of page table updates
> > requested by the guest OS. I simply limit it to only process one update:
> >
> > << for ( i = 0; i < count; i++ )
> >
> >>> for ( i= 0; i < 1; i++)
> >
> > However, after I install this version, there is a kernel panic.
> >
> > An alternative may be to modify the code in the paravirtualized guest OS
> > code, but I prefer to change xen code. Or do I HAVE to change the OS code
> > rather than xen?

I think you will need to change the OS code to unbatch the hypercalls.
In the example above you changed the semantics of the hypercall and when
the guest saw the unexpected return code (indicating that only 1/N
entries was processed) it -- rightly -- panic()d.

Ian.

> >
> > Could somebody provide some hints? Thanks!
> >
> > Qiang.
> >
> > _______________________________________________
> > Xen-users mailing list
> > Xen-users@xxxxxxxxxxxxxxxxxxx
> > http://lists.xensource.com/xen-users
> >
> 
> 
> 



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