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From: Peter Maydell <peter.maydell@linaro.org>
Date: Mon, 5 Aug 2013 13:01:14 +0100
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Subject: Re: [Qemu-devel] [PATCH v4 2/2] ARM: Add 'virt' platform
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To: Anup Patel <anup@brainfault.org>
Cc: "patches@linaro.org" <patches@linaro.org>, qemu-devel@nongnu.org, "kvmarm@lists.cs.columbia.edu" <kvmarm@lists.cs.columbia.edu>

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.

> 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.

thanks
-- PMM