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[Xen-devel] Re: [PATCH] paravirt/xen: add pvop for page_is_ram

To: "H. Peter Anvin" <hpa@xxxxxxxxx>
Subject: [Xen-devel] Re: [PATCH] paravirt/xen: add pvop for page_is_ram
From: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Date: Tue, 10 Mar 2009 15:44:52 -0700
Cc: Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>, the arch/x86 maintainers <x86@xxxxxxxxxx>, Linux Kernel Mailing List <linux-kernel@xxxxxxxxxxxxxxx>, Jeremy Fitzhardinge <jeremy.fitzhardinge@xxxxxxxxxx>
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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.


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