[Xen-devel] Re: [PATCH, experimental] i386 Allow the fixmap to be relocated at boot time

[Chris Wright <chrisw@xxxxxxxx>]

* Zachary Amsden (zach@xxxxxxxxxx) wrote:
> Your patch looks good, although the minimal change to subarch is to 
> merely have __FIXADDR_TOP defined by the sub-architecture.  Is there any 
> additional benefit to moving the fixmaps into the subarch - i.e. moving 
> subarch-specific pieces out of mach-default?

As you've identified, so subarch can put whatever wonky junk
it needs in there.

> I guess there is.  For example include/asm-i386/mach-visws/mach_fixmap.h 
> could do this:
> 
> #define SUBARCH_FIXMAPS \
>        FIX_CO_CPU,     /* Cobalt timer */ \
>        FIX_CO_APIC,    /* Cobalt APIC Redirection Table */ \
>        FIX_LI_PCIA,    /* Lithium PCI Bridge A */ \
>        FIX_LI_PCIB,    /* Lithium PCI Bridge B */
> 
> Then include/asm-i386/fixmap.h includes <mach_fixmap.h>, for which the 
> default is an empty define for SUBARCH_FIXMAPS.

nice.

> Also, it seems reasonable that people may want to poke holes in high 
> linear space for other hypervisor projects, research, or performance 
> reasons without having to build a custom sub-architecture just for 
> that.  So I think there is some benefit to making the hole size a 
> general configurable option (with defaults depending on the sub-arch you 
> select).

It needs to have tangible value for in-tree code.  Seems worthwhile to
play with it a bit though.

thanks,
-chris

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