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

[Auger Eric <eric.auger@redhat.com>]

Hi Laszlo,

On 05/24/2018 03:59 PM, Laszlo Ersek wrote:
> 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.

Is it what happens with the FW you provided to me? There is no LPAE in it?

> Previously, you wouldn't specify highmem=off, and things would just work
> -- the firmware would simply ignore the >=4GB MMIO apertur  e, 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.
Yes it is not possible to declare several disjoint ECAM spaces for a
single segment I think, hence the move.

 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);

Looks sensible to me

Thanks

Eric
> 
> Thanks,
> Laszlo
>