linux-kernel - Re: Introducing paravirt_ops for x86

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Date:	Wed, 08 Aug 2007 22:35:21 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Glauber de Oliveira Costa <glommer@...il.com>
CC:	"Nakajima, Jun" <jun.nakajima@...el.com>, lguest@...abs.org,
	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org, chrisw@...s-sol.org,
	anthony@...emonkey.ws, akpm@...ux-foundation.org,
	Glauber de Oliveira Costa <gcosta@...hat.com>,
	mingo@...e.hu
Subject: Re: Introducing paravirt_ops for x86_64

Glauber de Oliveira Costa wrote:
> On 8/8/07, Nakajima, Jun <jun.nakajima@...el.com> wrote:
>   
>> Glauber de Oliveira Costa wrote:
>>     
>>> Hi folks,
>>>
>>> After some time away from it, and a big rebase as a consequence, here
>>>       
>> is
>>     
>>> the updated version of paravirt_ops for x86_64, heading to inclusion.
>>>
>>> Your criticism is of course, very welcome.
>>>
>>> Have fun
>>>       
>> Do you assume that the kernel ougtht to use 2MB pages for its mappings
>> (e.g. initilal text/data,  direct mapping of physical memory) under your
>> paravirt_ops?  As far as I look at the patches, I don't find one.
>>     
>
> I don't think how it could be relevant here. lguest kernel does use
> 2MB pages, and it goes smootly. For 2MB pages, we will update the page
> tables in the very same way, and in the very places we did before.
> Just that the operations can now be overwritten.
>
> So, unless I'm very wrong,  it only makes sense to talk about not
> supporting large pages in the guest level. But it is not a
> paravirt_ops problem.
>   

At the moment Xen can't support guests with 2M pages.  In 32-bit this
isn't a huge problem, since the kernel doesn't assume it can map itself
with 2M pages.  But I think the 64-bit kernel assumes 2M pages are
always available for mapping the kernel.  I don't know how pervasive
this assumption is, but it would be nice to parameterize it in pv-ops.


    J
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