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

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Message-ID: <49B6ED64.2090007@goop.org>
Date:	Tue, 10 Mar 2009 15:44:52 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	"H. Peter Anvin" <hpa@...or.com>
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

H. Peter Anvin wrote:
>> 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?
>   

Well, on reflection, given that the thing we're testing for is "is page 
is allowed to be mapped by /dev/mem?", and devmem_is_allowed() already 
exists for precisely that reason, the answer is to put the hook there...

But, it seems I got the logic wrong anyway.  /dev/mem doesn't allow RAM 
pages to be mapped anyway, so granted pages masquerading as RAM will not 
be mappable via /dev/mem.  So I think we can safely drop this patch with 
no futher ado.

    J
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