Re: [Xen-devel] [RFC 15/19] xen/arm: Reserve region in guest memory for device passthrough

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Wed, 18 Jun 2014, Ian Campbell wrote:
> On Wed, 2014-06-18 at 16:23 +0100, Julien Grall wrote:
> > On 06/18/2014 04:12 PM, Stefano Stabellini wrote:
> > > On Mon, 16 Jun 2014, Julien Grall wrote:
> > >> This region will be split by the toolstack to allocate MMIO range for eac
> > >> device.
> > >>
> > >> For now only reserve a 512MB region, this should be enought to 
> > >> passthrough
> > >> multiple device at the same time.
> > >>
> > >> Signed-off-by: Julien Grall <julien.grall@xxxxxxxxxx>
> > >> ---
> > >>  xen/include/public/arch-arm.h |    4 ++++
> > >>  1 file changed, 4 insertions(+)
> > >>
> > >> diff --git a/xen/include/public/arch-arm.h 
> > >> b/xen/include/public/arch-arm.h
> > >> index ac54cd6..789bffb 100644
> > >> --- a/xen/include/public/arch-arm.h
> > >> +++ b/xen/include/public/arch-arm.h
> > >> @@ -369,6 +369,10 @@ typedef uint64_t xen_callback_t;
> > >>  #define GUEST_GICC_BASE   0x03002000ULL
> > >>  #define GUEST_GICC_SIZE   0x00000100ULL
> > >>  
> > >> +/* Space for mapping MMIO from device passthrough: 512MB @ 256MB*/
> > >> +#define GUEST_MMIO_BASE   0x10000000ULL
> > >> +#define GUEST_MMIO_SIZE   0x20000000ULL
> > > 
> > > Is it really necessary to specify size here? It looks like an artifical
> > > limitation to me: given that is unlikely that we'll ever be able to
> > > support non-PCI device hotplug, we only have to handle cold-plug here.
> > > So the toolstack has all the information it needs to build the perfect
> > > memory layout for the guest at VM creation time.
> > 
> > We have the same "artificial" limitation for the RAM banks... The
> > toolstack doesn't know where the different regions end up without the
> > size. As the layout may move in the future, adding the size avoid
> > modifying the toolstack every time we change it.
> 
> It also provides a documentary clue to people modifying things in this
> file to remind them to think about how much space they need to try and
> leave for this when adding something else.

I am not against documentation or resonable defaults. Let me explain
what I mean more clearly: if we are trying to assign 1 device with an
MMIO region of 1024MB, we know that it is not going to fit.

Can we rearrange the guest memory layout to increase GUEST_MMIO_SIZE?
After all the guest hasn't booted yet.

Otherwise could we calculate the size of the MMIO hole needed earlier,
at the time of building the guest p2m? This sounds harder.

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