linux-kernel - Re: [PATCH] paravirt/xen: add pvop for page_is

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Date:	Tue, 10 Mar 2009 15:21:31 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Xen-devel <xen-devel@...ts.xensource.com>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>
Subject: Re: [PATCH] paravirt/xen: add pvop for page_is_ram

Jeremy Fitzhardinge wrote:
>>
>> Why are these pages mapped as RAM in the memory map?  That is the right
>> way to handle that, not by adding yet another bloody hook...
>>   
> Granted pages can turn up anywhere dynamically, since they're pages
> borrowed from other domains for the purposes of IO.  They're not static
> regions of non-RAM like the other cases page_is_ram() tests for,
> 
> They can't be mapped via normal pte operations (because they have
> additional state associated with them, like the grant handle), so
> /dev/mem can't just create an aliased mapping by copying the pte.
> 
> page_is_ram is used to:
> 
>   1. prevent /dev/mem from mapping non-RAM pages
>   2. prevent ioremap from mapping any RAM pages
>   3. testing for RAMness in PAT
> 
> 3) isn't yet relevant to Xen; ioremap can't map granted pages either, so
> 2) isn't terribly relevent, so the main motivation for this patch is
> 1).  This allows us to reject usermode attempts to map granted pages,
> rather than oopsing (as a failed set_pte will raise a page fault).
> 
> So, more cosmetic than essential, but I don't see a better way to
> implement this functionality if its to be there at all.
> 

OK, that is a valid usage case and I agree about repurposing the
existing interface.  However, it is also a definition change in the
interface, so it really should be renamed first.

Would you be willing to break this patch up into one which renames the
interface and then a second which adds the pv hook?

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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