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

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Message-ID: <YPWAeTJMWJ+A7W2c@google.com>
Date:   Mon, 19 Jul 2021 14:39:05 +0100
From:   Quentin Perret <qperret@...gle.com>
To:     Marc Zyngier <maz@...nel.org>
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 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.

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.

Thanks,
Quentin