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

[Julien Grall <julien.grall@xxxxxxxxxx>]

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.

-- 
Julien Grall

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