Re: [Qemu-devel] [PATCH v4 2/2] ARM: Add 'virt' platform

[Anup Patel <anup@brainfault.org>]

On Mon, Aug 5, 2013 at 5:58 PM, Peter Maydell <peter.maydell@linaro.org> wrote:
> On 5 August 2013 13:22, Anup Patel <anup@brainfault.org> wrote:
>> On Mon, Aug 5, 2013 at 5:31 PM, Peter Maydell <peter.maydell@linaro.org> wrote:
>>> On 5 August 2013 12:48, Anup Patel <anup@brainfault.org> wrote:
>>>>> +static const MemMapEntry a15memmap[] = {
>>>>> +    [VIRT_FLASH] = { 0, 0x100000 },
>>>>
>>>> IMHO, 1 MB of flash is small for possible future expansion. If mach-virt
>>>> becomes popular then we can expect people running UBoot or UEFI or
>>>> .... from this flash.
>>>>
>>>> I think having 16 MB of flash would be good because it is multiple of
>>>> 2 MB hence we can also create section entries for it in Stage2 TTBL.
>>>
>>> Seems reasonable.
>>>
>>>>> +    [VIRT_CPUPERIPHS] = { 0x100000, 0x8000 },
>>>>
>>>> I would suggest to start peripheral space at 2 MB boundary and also
>>>> have its total size in multiple of 2 MB.
>>>
>>> The total size here is fixed by the CPU hardware -- an A15's
>>> private peripheral space is only 0x8000 in size.
>>
>> Does this mean, mach-virt address space is Cortex-A15 specific ?
>
> The patch has support for an array of VirtBoardInfo structs,
> each of which has its own memmap. The only entry at the
> moment is for the A15.
>
>> What about Cortex-A7 and Cortex-A12  ?
>
> QEMU supports neither of these CPUs. The A7's the same as
> the A15, though.
>
>> If above is a mandatory constraint then we can have peripheral space divide
>> into two parts:
>> 1. Essential (or MPCore) peripherals: For now only GIC belongs here. Other
>> things that fit here is Watchdog and Local timers but this are redundant for
>> mach-virt. This space can be only 0x8000 in size as required by Cortex-A15.
>> 2. General peripherals: All VirtIO devices would belong here. In addition,
>> users can also add devices to this space using QEMU command line.
>
> Er, this is what the patch already does. The "CPUPERIPHS"
> bit is specifically for the CPU's private peripheral region.
>
> You can't add an MMIO device on the QEMU command line, there's
> no way to wire up the memory regions and IRQs.
>
>>> Why does each individual peripheral have to be at a 2MB boundary?
>>> I would expect the kernel to just have a single "nothing mapped"
>>> here stage 2 table entry (or entries) covering the whole of the
>>> mmio peripheral region regardless of how many devices happen
>>> to be inside it.
>>
>> Yes, this seem a little over-kill but we can atleast have peripheral space to
>> be 2MB aligned with total size in multiple of 2MB.
>
> I really don't want to eat 2MB for each virtio-mmio transport
> in a 32 bit address space, it seems hugely wasteful unless
> there's a good reason to do it.

I am not suggesting to give 2MB space to each virtio-mmio transport.

What I really meant was to start VIRT_MMIO space (where all the
virtio-mmio transport would be added) to start at 2MB aligned address
and have total size (include all virtio-mmio transports) to be in-multiple
of 2MB so that we can trap access to all virtio-mmio transports using
1-2 2MB entries in Stage2.

>
> -- PMM

--Anup