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[Xen-devel] Re: [PATCH] x86: define arch_vm_get_page_prot to set _PAGE_I

To: "H. Peter Anvin" <hpa@xxxxxxxxx>
Subject: [Xen-devel] Re: [PATCH] x86: define arch_vm_get_page_prot to set _PAGE_IOMAP on VM_IO vmas
From: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Date: Fri, 22 Oct 2010 12:36:43 -0700
Cc: "Xen-devel@xxxxxxxxxxxxxxxxxxx" <Xen-devel@xxxxxxxxxxxxxxxxxxx>, the arch/x86 maintainers <x86@xxxxxxxxxx>, Linux Kernel Mailing List <linux-kernel@xxxxxxxxxxxxxxx>, Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
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 On 10/22/2010 12:20 PM, H. Peter Anvin wrote:
> On 10/22/2010 12:06 PM, Jeremy Fitzhardinge wrote:
>> Well, if you want to map a normal memory page, you'd use, say,
>> pfn_pte(pfn, PAGE_KERNEL) to generate the pte.  The pfn is a
>> domain-local pseudo-physical address.  When it ends up in
>> xen_make_pte(), it will translate the the pfn into a machine-global mfn
>> to generate a pte_t which can be inserted into a pagetable.  (And when
>> that pagetable starts being used as such, Xen will validate that the mfn
>> is actually one the domain is allowed to address.)
>> However, if you're doing an ioremap(), then the mapped address is a
>> hardware one.  In that case, we construct the pte with
>> pfn_pte(device_pfn, PAGE_KERNEL_IO), which sets the _PAGE_IOMAP flag in
>> the pte flags.  When it gets to xen_make_pte(), it sees _PAGE_IOMAP and
>> constructs a pte_t containing the literal untranslated device_pfn
>> (really an mfn).  (And again, Xen will check that the domain has access
>> to that mfn before allowing the mapping to be used.)
> When you're doing an ioremap(), then the mapped address is *both* a PFN
> and an MFN, right?  So why do your need a flag?  That is the part I
> don't get...

Xen always needs an mfn, so for memory the pfn needs to be converted to
an mfn, but for devices the frame number is already an mfn.  You could
look at it as having two distinct frame number address spaces, and the
_PAGE_IOMAP flag tells the lower levels which address space the frame
number is in.  Both memory and device mappings use the same interface -
__pte() - to convert a (pfn,prot) tuple into a pte_t, so we use a bit in
prot to distinguish them.

In the kernel, the places where device mappings vs memory mappings are
quite well delineated.  There used to be a few places which could be
either depending on context, but they've since been cleaned up by other
efforts (the PAT/pageattr work did a lot of them, I think).


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