linux-kernel - Re: [PATCH 03/16] KVM: arm64: Turn kvm_pgtable_stage2_set_owner into kvm_pgtable_stage2

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Message-ID: <YPanmXfdr9rqnICK@google.com>
Date:   Tue, 20 Jul 2021 11:38:17 +0100
From:   Quentin Perret <qperret@...gle.com>
To:     Marc Zyngier <maz@...nel.org>
Cc:     linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org, will@...nel.org,
        dbrazdil@...gle.com, Srivatsa Vaddagiri <vatsa@...eaurora.org>,
        Shanker R Donthineni <sdonthineni@...dia.com>,
        James Morse <james.morse@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        kernel-team@...roid.com
Subject: Re: [PATCH 03/16] KVM: arm64: Turn kvm_pgtable_stage2_set_owner into
 kvm_pgtable_stage2_annotate

On Tuesday 20 Jul 2021 at 11:21:17 (+0100), Marc Zyngier wrote:
> On Tue, 20 Jul 2021 11:09:21 +0100,
> Quentin Perret <qperret@...gle.com> wrote:
> > 
> > On Thursday 15 Jul 2021 at 17:31:46 (+0100), Marc Zyngier wrote:
> > > @@ -815,7 +807,7 @@ int kvm_pgtable_stage2_set_owner(struct kvm_pgtable *pgt, u64 addr, u64 size,
> > >  		.arg		= &map_data,
> > >  	};
> > >  
> > > -	if (owner_id > KVM_MAX_OWNER_ID)
> > > +	if (!annotation || (annotation & PTE_VALID))
> > >  		return -EINVAL;
> > 
> > Why do you consider annotation==0 invalid? The assumption so far has
> > been that the owner_id for the host is 0, so annotating a range with 0s
> > should be a valid operation -- this will be required when e.g.
> > transferring ownership of a page back to the host.
> 
> How do you then distinguish it from an empty entry that doesn't map to
> anything at all?

You don't, but that's beauty of it :)

The host starts with a PGD full of zeroes, which in terms of ownership
means that it owns the entire (I)PA space. And it loses ownership of a
page only when we explicitly annotate it with an owner id != 0.