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

From: Marc Zyngier <maz@kernel.org>
To: Peter Maydell <peter.maydell@linaro.org>
Cc: kvm-devel <kvm@vger.kernel.org>,
	QEMU Developers <qemu-devel@nongnu.org>,
	Android Kernel Team <kernel-team@android.com>,
	kvmarm <kvmarm@lists.cs.columbia.edu>
Subject: Re: [PATCH 3/3] docs/system/arm/virt: Fix documentation for the 'highmem' option
Date: Wed, 08 Sep 2021 10:16:47 +0100	[thread overview]
Message-ID: <877dfrctq8.wl-maz@kernel.org> (raw)
In-Reply-To: <CAFEAcA_MRyd2AcgAhvEwJY8LGbHoyz_JgTdMGAEtGegvZB0d7A@mail.gmail.com>

On Tue, 07 Sep 2021 19:25:23 +0100,
Peter Maydell <peter.maydell@linaro.org> wrote:
> 
> 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 ?

Exactly. I have the gut feeling that we need a 'gpa_bits' option that
would limit the guest physical range and generalise highmem. High IO
ranges would simply not be available if the GPA range isn't big
enough.

	M.

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