Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Tue, 20 Jul 2021 09:46:51 +0100
Message-ID: <874kcp9xtw.wl-maz@kernel.org>
From:   Marc Zyngier <maz@kernel.org>
To:     Quentin Perret <qperret@google.com>
Cc:     james.morse@arm.com, alexandru.elisei@arm.com,
        suzuki.poulose@arm.com, catalin.marinas@arm.com, will@kernel.org,
        linux-arm-kernel@lists.infradead.org, kvmarm@lists.cs.columbia.edu,
        linux-kernel@vger.kernel.org, ardb@kernel.org, qwandor@google.com,
        tabba@google.com, dbrazdil@google.com, kernel-team@android.com
Subject: Re: [PATCH 05/14] KVM: arm64: Don't overwrite ignored bits with owner id
In-Reply-To: <YPWAeTJMWJ+A7W2c@google.com>
References: <20210719104735.3681732-1-qperret@google.com>
        <20210719104735.3681732-6-qperret@google.com>
        <87im16jwe6.wl-maz@kernel.org>
        <YPWAeTJMWJ+A7W2c@google.com>
User-Agent: Wanderlust/2.15.9 (Almost Unreal) SEMI-EPG/1.14.7 (Harue)
 FLIM-LB/1.14.9 (=?UTF-8?B?R29qxY0=?=) APEL-LB/10.8 EasyPG/1.0.0 Emacs/27.1
 (x86_64-pc-linux-gnu) MULE/6.0 (HANACHIRUSATO)
MIME-Version: 1.0 (generated by SEMI-EPG 1.14.7 - "Harue")
Content-Type: text/plain; charset=US-ASCII
Precedence: bulk

On Mon, 19 Jul 2021 14:39:05 +0100,
Quentin Perret <qperret@google.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@google.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.