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

[Jeremy Fitzhardinge <jeremy@goop.org>]

Avi Kivity wrote:
> Jeremy Fitzhardinge wrote:
>> Use one of the software-defined PTE bits to indicate that a mapping is
>> intended for an IO address.  On native hardware this is irrelevent,
>> since a physical address is a physical address.  But in a virtual
>> environment, physical addresses are also virtualized, so there needs
>> to be some way to distinguish between pseudo-physical addresses and
>> actual hardware addresses; _PAGE_IOMAP indicates this intent.
>>
>> By default, __supported_pte_mask masks out _PAGE_IOMAP, so it doesn't
>> even appear in the final pagetable.
>>   
>
> Could PTE_SPECIAL, added for get_user_pages_really_fast(), be reused
> for this?
>

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?

Also, _PAGE_SPECIAL is also shared with _PAGE_CPA_TEST, which is only
used on kernel mappings, so they can co-exist happily.

Is _PAGE_IOMAP at all useful for device passthrough in kvm?

    J