Re: [Xen-devel] Re: [PATCH 1 of 7] x86: add _PAGE_IOMAP pte flag for IO mappings

Keir Fraser
2008-09-09T15:29:21Z

On 9/9/08 15:56, "Avi Kivity" <avi@qumranet.com> wrote:

>> I'm not sure; I still don't really understand how _PAGE_SPECIAL gets
>> used, other than being user-mode mapping only. But in principle,
>> _PAGE_IOMAP could be set on both kernel and user mappings (if you direct
>> map a device into a process address space), so I think they would
>> conflict then?
>
> It's a "don't refcount me" flag, which is not sematically the same as
> I/O, but may be close enough.

That's basically what our _PAGE_IO flag (in our old Linux patchset) means.
We use it to cause pte_pfn() to return an invalid pfn and hence avoid
reference counting that way. Since kernel mappings are never reference
counted (I think?) perhaps we could use _PAGE_SPECIAL even if it is
restricted to use on user mappings.

-- Keir