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, Julien Grall wrote:
> On 06/18/2014 06:48 PM, Stefano Stabellini wrote:
> > 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.
> 
> For non-PCI passthrough this size will unlikely happen. We always map a
> matter of few pages per-device.
> 
> I think this size is enough for the time being. I plan to revisit it for
> PCI passthrough where we will be able to allocate some of them after 4G.

Modern GPUs can easily exceed 512MB and they work on ARM.

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