Re: [PATCH 3/3] docs/system/arm/virt: Fix documentation for the 'highmem' option

[Marc Zyngier]

Hi Peter,

On Tue, 07 Sep 2021 13:51:13 +0100,
Peter Maydell <peter.maydell@linaro.org> wrote:
> 
> On Sun, 22 Aug 2021 at 15:45, Marc Zyngier <maz@kernel.org> wrote:
> >
> > The documentation for the 'highmem' option indicates that it controls
> > the placement of both devices and RAM. The actual behaviour of QEMU
> > seems to be that RAM is allowed to go beyond the 4GiB limit, and
> > that only devices are constraint by this option.
> >
> > Align the documentation with the actual behaviour.
> 
> I think it would be better to align the behaviour with the documentation.
> 
> The intent of 'highmem' is to allow a configuration for use with guests
> that can't address more than 32 bits (originally, 32-bit guests without
> LPAE support compiled in). It seems like a bug that we allow the user
> to specify more RAM than will fit into that 32-bit range. We should
> instead make QEMU exit with an error if the user tries to specify
> both highmem=off and a memory size that's too big to fit.

I'm happy to address this if you are OK with the change in user
visible behaviour.

However, I am still struggling with my original goal, which is to
allow QEMU to create a usable KVM_based VM on systems with a small IPA
space (36 bits on the system I have). What would an acceptable way to
convey this to the code that deals with the virt memory map so that it
falls back to something that actually works?

Thanks,

        M.

-- 
Without deviation from the norm, progress is not possible.