Re: [PULL 18/38] hw/arm/virt: Honor highmem setting when computing the memory map

[Peter Maydell]

On Sun, 13 Feb 2022 at 10:22, Marc Zyngier <maz@kernel.org> wrote:
>
> [+ Alex for HVF]
>
> On Sun, 13 Feb 2022 05:05:33 +0000,
> Akihiko Odaki <akihiko.odaki@gmail.com> wrote:
> > Hi,
> > This breaks in a case where highmem is disabled but can have more than
> > 4 GiB of RAM. M1 (Apple Silicon) actually can have 36-bit PA with HVF,
> > which is not enough for highmem MMIO but is enough to contain 32 GiB
> > of RAM.
>
> Funny. The whole point of this series is to make it all work correctly
> on M1.
>
> > Where the magic number of 4 GiB / 32-bit came from?
>
> Not exactly a magic number. From QEMU's docs/system/arm/virt.rst:
>
> <quote>
> highmem
>   Set ``on``/``off`` to enable/disable placing devices and RAM in physical
>   address space above 32 bits. The default is ``on`` for machine types
>   later than ``virt-2.12``.
> </quote>
>
> TL;DR: Removing the bogus 'highmem=off' option from your command-line
> should get you going with large memory spaces, up to the IPA limit.

Yep. I've tested this with hvf, and we now correctly:
 * refuse to put RAM above 32-bits if you asked for a 32-bit
   IPA space with highmem=off
 * use the full 36-bit address space if you don't say highmem=off
   on an M1

Note that there is a macos bug where if you don't say highmem=off
on an M1 Pro then you'll get a macos kernel panic. M1 non-Pro is fine.

thanks
-- PMM