Re: [Qemu-devel] [RFC 0/2] ARM virt: Support up to 256 PCIe buses

[Laszlo Ersek <lersek@redhat.com>]

On 05/24/18 15:07, Peter Maydell wrote:
> On 24 May 2018 at 13:59, Laszlo Ersek <lersek@redhat.com> wrote:
>> On 05/24/18 11:11, Peter Maydell wrote:
>>> Won't it also break a guest which is just Linux loaded not via
>>> firmware which is an aarch32 kernel without LPAE support?
>>
>> Does such a thing exist? (I honestly have no clue.)
> 
> Yes, it does; LPAE isn't a mandatory kernel config option.
> This is why we have the machine 'highmem' option, so that
> we can run on those kernels by not putting anything above
> the 4G boundary. Looking back at the history on that, we
> opted at the time for "default to highmem on, and if you're
> running an non-lpae kernel you need to turn it off manually".

Ah, OK, I didn't know that.

> So we can handle those kernels by just not putting ECAM
> above 4G if highmem is false.

The problem is we can have a combination of 32-bit UEFI firmware (which
certainly lacks LPAE) and a 32-bit kernel which supports LPAE.
Previously, you wouldn't specify highmem=off, and things would just work
-- the firmware would simply ignore the >=4GB MMIO aperture, and use the
32-bit MMIO aperture only (and use the sole 32-bit ECAM). The kernel
could then use both low and high MMIO apertures, however (I gather?).

The difference with "high ECAM" is that it is *moved* (not *added*), so
the 32-bit firmware is left with nothing for config space access. For
booting the same combination as above, you are suddenly forced to add
highmem=off, just to keep the ECAM low -- and that, while it keeps the
firmware happy, prevents the LPAE-capable kernel from using the high
MMIO aperture.

So I think "highmem_ecam" should be computed like this:

  highmem_ecam = highmem_ecam_machtype_default &&
                 highmem &&
                 (!firmware_loaded || aarch64);

Thanks,
Laszlo