Xen devel - Oct 2012 - Sharing MMIO page with DomU on a system with no IOMMU

Hi -
I''m trying to share an MMIO page from Dom0 running linux with a DomU 
running an RTOS so that I can access the GPIO registers directly. The 
system I need to get this working on does not have an IOMMU. I''m aware 
of the security issues of sharing this area, but don''t believe
it''s an
issue in this case.  I''ve got a Linux module set up and can share
memory
between the two domains using the grant table, but I haven''t been able 
to get this to work for the MMIO page.

Can anyone give me a pointer on how I should go about doing this?  I 
need the area to actually be shared between both domains, not simply 
owned by the RTOS.


Thanks.
Martin Roth

>>> On 15.10.12 at 18:50, Martin Roth <gaumless@gmail.com> wrote:
> I''m trying to share an MMIO page from Dom0 running linux with a
DomU
> running an RTOS so that I can access the GPIO registers directly. The 
> system I need to get this working on does not have an IOMMU. I''m
aware
> of the security issues of sharing this area, but don''t believe
it''s an
> issue in this case.  I''ve got a Linux module set up and can share
memory
> between the two domains using the grant table, but I haven''t been
able
> to get this to work for the MMIO page.
> 
> Can anyone give me a pointer on how I should go about doing this?  I 
> need the area to actually be shared between both domains, not simply 
> owned by the RTOS.
Looks like you have to assign the PCI device providing the GPIOs
to the guest, as there doesn''t currently appear to be a way to
allow guest access to individual MMIO pages (lacking the MMIO
counterpart to the iports= and irqs= domain config settings).

Ian, Ian - am I overlooking something?

And of course - sharing the MMIO page between Dom0 and guest
is a questionable intention in any case - you rather want it owned
by the DomU, and Dom0 then shouldn''t access it at all. Otherwise,
how would you expect accesses to be coordinated?

Jan

On Tue, 2012-10-16 at 08:08 +0100, Jan Beulich wrote:
> >>> On 15.10.12 at 18:50, Martin Roth <gaumless@gmail.com>
wrote:
> > I''m trying to share an MMIO page from Dom0 running linux with
a DomU
> > running an RTOS so that I can access the GPIO registers directly. The 
> > system I need to get this working on does not have an IOMMU.
I''m aware
> > of the security issues of sharing this area, but don''t
believe it''s an
> > issue in this case.  I''ve got a Linux module set up and can
share memory
> > between the two domains using the grant table, but I haven''t
been able
> > to get this to work for the MMIO page.
> > 
> > Can anyone give me a pointer on how I should go about doing this?  I 
> > need the area to actually be shared between both domains, not simply 
> > owned by the RTOS.
> 
> Looks like you have to assign the PCI device providing the GPIOs
> to the guest, as there doesn''t currently appear to be a way to
> allow guest access to individual MMIO pages (lacking the MMIO
> counterpart to the iports= and irqs= domain config settings).
> 
> Ian, Ian - am I overlooking something?
iomem= support was committed to xl in xen-unstable last week or the week
before. I think it already existed in xend.
> 
> And of course - sharing the MMIO page between Dom0 and guest
> is a questionable intention in any case - you rather want it owned
> by the DomU, and Dom0 then shouldn''t access it at all. Otherwise,
> how would you expect accesses to be coordinated?
> 
> Jan
>

>>> On 16.10.12 at 09:19, Ian Campbell <Ian.Campbell@citrix.com>
wrote:
> On Tue, 2012-10-16 at 08:08 +0100, Jan Beulich wrote:
>> >>> On 15.10.12 at 18:50, Martin Roth
<gaumless@gmail.com> wrote:
>> > I''m trying to share an MMIO page from Dom0 running linux
with a DomU
>> > running an RTOS so that I can access the GPIO registers directly.
The
>> > system I need to get this working on does not have an IOMMU.
I''m aware
>> > of the security issues of sharing this area, but don''t
believe it''s an
>> > issue in this case.  I''ve got a Linux module set up and
can share memory
>> > between the two domains using the grant table, but I
haven''t been able
>> > to get this to work for the MMIO page.
>> > 
>> > Can anyone give me a pointer on how I should go about doing this? 
I
>> > need the area to actually be shared between both domains, not
simply
>> > owned by the RTOS.
>> 
>> Looks like you have to assign the PCI device providing the GPIOs
>> to the guest, as there doesn''t currently appear to be a way to
>> allow guest access to individual MMIO pages (lacking the MMIO
>> counterpart to the iports= and irqs= domain config settings).
>> 
>> Ian, Ian - am I overlooking something?
> 
> iomem= support was committed to xl in xen-unstable last week or the week
> before. I think it already existed in xend.
I fail to spot anything like that in xend - the only use of
domain_iomem_permission() is in server\pciif.py afaics.

Jan

On Tue, 2012-10-16 at 08:55 +0100, Jan Beulich wrote:
> >>> On 16.10.12 at 09:19, Ian Campbell
<Ian.Campbell@citrix.com> wrote:
> > On Tue, 2012-10-16 at 08:08 +0100, Jan Beulich wrote:
> >> >>> On 15.10.12 at 18:50, Martin Roth
<gaumless@gmail.com> wrote:
> >> > I''m trying to share an MMIO page from Dom0 running
linux with a DomU
> >> > running an RTOS so that I can access the GPIO registers
directly. The
> >> > system I need to get this working on does not have an IOMMU.
I''m aware
> >> > of the security issues of sharing this area, but
don''t believe it''s an
> >> > issue in this case.  I''ve got a Linux module set up
and can share memory
> >> > between the two domains using the grant table, but I
haven''t been able
> >> > to get this to work for the MMIO page.
> >> > 
> >> > Can anyone give me a pointer on how I should go about doing
this?  I
> >> > need the area to actually be shared between both domains, not
simply
> >> > owned by the RTOS.
> >> 
> >> Looks like you have to assign the PCI device providing the GPIOs
> >> to the guest, as there doesn''t currently appear to be a
way to
> >> allow guest access to individual MMIO pages (lacking the MMIO
> >> counterpart to the iports= and irqs= domain config settings).
> >> 
> >> Ian, Ian - am I overlooking something?
> > 
> > iomem= support was committed to xl in xen-unstable last week or the
week
> > before. I think it already existed in xend.
> 
> I fail to spot anything like that in xend - the only use of
> domain_iomem_permission() is in server\pciif.py afaics.
You are right. I saw iomem in the grep output and neglected to look at
the path.

Ian.