Xen devel - Jun 2007 - help to understand is_running _on_xen ?

help to understand is_running _on_xen ?

   I don''t  understand why we check this condition is_running_on_xen 
?In all drivers(front & back) code we are checking this condition ?what
is this function stands for ? what it does ?


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> -----Original Message-----
> From: xen-devel-bounces@lists.xensource.com 
> [mailto:xen-devel-bounces@lists.xensource.com] On Behalf Of 
> Gautham Kampalapur Shankar, TLS, Chennai
> Sent: 04 June 2007 16:05
> To: xen-devel@lists.xensource.com
> Subject: [Xen-devel] help to understand is_running _on_xen ?
> 
> 
>    I don''t  understand why we check this condition 
> is_running_on_xen  ?In all drivers(front & back) code we are 
> checking this condition ?what is this function stands for ? 
> what it does ?
If you search for "is_running_on_xen" in the Linux tree,
you''ll find
that the x86 definition of this is "1" - so any test for this,
currently, means that it''s "true". IA64 is different, at
least in the
exact implementation. 

I can''t explain this in any othe way than that the plan is to allow for
drivers to check if they are running on a native platform or running on
top of Xen, and make decisions (such as "not attempt to make
hypercalls") based on which case it is - so if we''re not running
on Xen,
don''t make hypercalls to Xen, because it wont work!

--
Mats
> 
> 


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> >    I don''t  understand why we check this condition
> > is_running_on_xen  ?In all drivers(front & back) code we are
> > checking this condition ?what is this function stands for ?
> > what it does ?
>
> If you search for "is_running_on_xen" in the Linux tree,
you''ll find
> that the x86 definition of this is "1" - so any test for this,
> currently, means that it''s "true". IA64 is different, at
least in the
> exact implementation.
>
> I can''t explain this in any othe way than that the plan is to
allow for
> drivers to check if they are running on a native platform or running on
> top of Xen, and make decisions (such as "not attempt to make
> hypercalls") based on which case it is - so if we''re not
running on Xen,
> don''t make hypercalls to Xen, because it wont work!
IA64, at least at one stage (not sure of the current status, since there have 
been quite a few changes since I read about this), supported "transparent 
paravirtualisation" whereby the same kernel could run with minimal 
performance loss both on the native hardware and paravirtualised.

The x86 paravirt_ops code for Linux is designed to allow this mode of 
operation for x86_32 systems also (and presumably, eventually it''ll
work for
x86_64).

It doesn''t make any sense to enable the Xen-aware drivers when not
running on
Xen - they won''t work as the platform features they require (hypercalls
etc)
are not present.

is_running_on_xen() may be defined to 1 (as in the linux-sparse tree supplied 
with Xen) if the kernel is only ever intended to run paravirtualised, or if 
the kernel is capable of transparent paravirtualisation it may call to a 
function which checks for the presence of Xen and returns nonzero only if Xen 
is available.

HTH,
Cheers,
Mark

-- 
Dave: Just a question. What use is a unicyle with no seat?  And no pedals!
Mark: To answer a question with a question: What use is a skateboard?
Dave: Skateboards have wheels.
Mark: My wheel has a wheel!

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