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

[Peter Maydell]

On Tue, 7 Sept 2021 at 18:10, Marc Zyngier <maz@kernel.org> wrote:
>
> 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?

Hmm, so at the moment we can either do "fits in 32 bits" or
"assumes at least 40 bits" but not 36 ?

thanks
-- PMM