linux-kernel - Re: [PATCH 05/14] KVM: arm64: Don't overwrite ignored bits with owner id

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Date:   Tue, 20 Jul 2021 09:46:51 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     Quentin Perret <qperret@...gle.com>
Cc:     james.morse@....com, alexandru.elisei@....com,
        suzuki.poulose@....com, catalin.marinas@....com, will@...nel.org,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        linux-kernel@...r.kernel.org, ardb@...nel.org, qwandor@...gle.com,
        tabba@...gle.com, dbrazdil@...gle.com, kernel-team@...roid.com
Subject: Re: [PATCH 05/14] KVM: arm64: Don't overwrite ignored bits with owner id

On Mon, 19 Jul 2021 14:39:05 +0100,
Quentin Perret <qperret@...gle.com> wrote:
> 
> On Monday 19 Jul 2021 at 13:55:29 (+0100), Marc Zyngier wrote:
> > On Mon, 19 Jul 2021 11:47:26 +0100,
> > Quentin Perret <qperret@...gle.com> wrote:
> > > 
> > > The nVHE protected mode uses invalid mappings in the host stage-2
> > > page-table to track the owner of each page in the system. In order to
> > > allow the usage of ignored bits (a.k.a. software bits) in these
> > > mappings, move the owner encoding away from the top bits.
> > 
> > But that's exactly what the current situation is allowing: the use of
> > the SW bits. I am guessing that what you really mean is that you want
> > to *preserve* the SW bits from an existing mapping and use other bits
> > when the mapping is invalid?
> 
> Yes, this is really just forward looking, but I think it might be useful
> to allow annotating invalid mappings with both an owner id _and_
> additional flags for e.g. shared pages and such. And using bits [58-55]
> to store those flags just like we do for valid mappings should make
> things easier, but no big deal.

Right, so maybe worth calling that out.

> I see how this is going to conflict with kvm_pgtable_stage2_annotate()
> from your series though, so maybe I should just drop this patch and
> leave the encoding 'issue' to the caller -- the rest of the series
> doesn't depend on this anyway, this was just small cleanup.

I'm not too worried about that for now. We can always rewrite one in
terms of the other, but I wanted to understand exactly this change was
about.

Thanks,

	M.

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