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Re: [Xen-ia64-devel] paravirt_ops and its alternatives

To: "Dong, Eddie" <eddie.dong@xxxxxxxxx>
Subject: Re: [Xen-ia64-devel] paravirt_ops and its alternatives
From: Isaku Yamahata <yamahata@xxxxxxxxxxxxx>
Date: Wed, 6 Feb 2008 10:59:37 +0900
Cc: "Luck, Tony" <tony.luck@xxxxxxxxx>, Alex Williamson <alex.williamson@xxxxxx>, xen-ia64-devel@xxxxxxxxxxxxxxxxxxx
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On Tue, Feb 05, 2008 at 10:17:10PM +0800, Dong, Eddie wrote:
>       1: The coding style is not as good as original IVT code.

I have to agree with you here.

>               For example:
> #ifdef CONFIG_XEN
>         mov r24=r8
>         mov r8=r18
>         ;;
> (p10)   XEN_HYPER_ITC_I
>         ;;
> (p11)   XEN_HYPER_ITC_D
>         ;;
>         mov r8=r24
>         ;;
> #else
>               This kind of save/restore R8 in each replacement (MACRO)
>       is kind of not well tuned. We probably need a big IVT code
> change
>       to avoid frequent save/restore in each MACRO.
>               This needs many effort. Of course taking shortcut before
> into upstream.

Yes, such register value save/restore is suboptimal.
I'm guessing such overhead is relatively small compared to
the hyperprivops overhead which issues break instruction.
So presumably for reducing such overhead, it is necessary to replace 
those break instructions with fast hyperprivops using gate page.
Such optimization would be the next step after upstream merge though.


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