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

[Anup Patel <anup@brainfault.org>]

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 ?
What about Cortex-A7 and Cortex-A12  ?

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.

>
>> This will enable us to create 2 MB
>> entries in Stage2 TTBL for trapping which in-turn can help in performance
>> by reducing Stage2 TTBL walks.
>>
>> I am sure there won't be too many peripherals in mach-virt so, it would
>> even better if we can have base address of each peripheral to be at
>> 2 MB boundary.
>
> 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.

>
> thanks
> -- PMM