Re: [PATCH 11/16] x86: add a boot option to enable and disable the direct map

[Hongyan Xia <hx242@xxxxxxx>]

On Fri, 2020-05-01 at 12:11 +0000, Wei Liu wrote:
> On Thu, Apr 30, 2020 at 09:44:20PM +0100, Hongyan Xia wrote:
> > From: Hongyan Xia <hongyxia@xxxxxxxxxx>
> > 
> > Also add a helper function to retrieve it. Change
> > arch_mfn_in_direct_map
> > to check this option before returning.
> > 
> > This is added as a boot command line option, not a Kconfig. We do
> > not
> > produce different builds for EC2 so this is not introduced as a
> > compile-time configuration.
> 
> Having a Kconfig will probably allow the compiler to eliminate dead
> code.
> 
> This is not asking you to do the work, someone can come along and
> adjust 
> arch_has_directmap easily.

My original code added this as a CONFIG option, but I converted it into
a boot-time switch, so I can just dig out history and convert it back.
I wonder if we should get more opinions on this to make a decision.

I would love Xen to have static key support though so that a boot-time
switch costs no run-time performance.

Hongyan