linux-kernel - Re: [RFC] Use para_fill instead of vmi_get

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Date:	Tue, 27 Feb 2007 10:17:19 -0600
From:	Anthony Liguori <anthony@...emonkey.ws>
To:	Zachary Amsden <zach@...are.com>
CC:	linux-kernel@...r.kernel.org, Jeremy Fitzhardinge <jeremy@...p.org>
Subject: Re: [RFC] Use para_fill instead of vmi_get_function for APIC ops

Zachary Amsden wrote:
> Anthony Liguori wrote:
>> Hi Zach,
>>
>> It seems to me that the APIC paravirt_ops should be filled by 
>> para_fill() instead of vmi_get_function().  vmi_get_function() 
>> returns a nop when the relocation type is NONE.  para_fill() leaves 
>> the native code in place.
>>
>> The native version of the apic write ops is more or less *(APIC_BASE 
>> + reg) = value.  APIC_BASE is unknown to the ROM so it's impossible 
>> to simulate this in the ROM.
>>
>> This means that a ROM has no choice but to do APIC emulation (or jump 
>> through seriously hairy loops to get the APIC mapped in it's address 
>> space).  Was this the intention?
>
> No, but certainly the effect.  Actually, it is very easy to get the 
> APIC mapped in the ROM address space without jumping through seriously 
> hairy loops - we do it today in our hypervisor.

I neglected to mention that I didn't want to use a memory hole.  One 
could allocate a small one to map the APIC but that seems to defeat the 
purpose of having a native ROM.

Regards,

Anthony Liguori

>>
>> N.B. attached patch is just to illustrate the point.  Has not even 
>> been compile tested.
>
>
> Patch looks good, thanks.   But the whole para_fill / vmi_get_function 
> stuff could probably be done even cleaner.  It was just a helper at 
> first to work around the awkward syntax, and it is still a bit ugly, 
> but I haven't come up with a better solution yet, mostly because with 
> the new inlining work Jeremy is doing, we might want to start doing 
> selective inlining, in which case I'll have to go back over the code 
> anyway to clean everything to get the logic right in all cases.
>
> I assume this patch is signed-off-by you?  If so, I'll add it to my 
> patch queue.
>
> Zach
>

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