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

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Date:   Tue, 20 Jul 2021 12:36:21 +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 12:20:58 (+0100), Marc Zyngier wrote:
> On Tue, 20 Jul 2021 11:38:17 +0100,
> Quentin Perret <qperret@...gle.com> wrote:
> > 
> > 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.
> 
> Right. But this scheme doesn't apply to the guests, does it?

Right, the meaning of a NULL PTE in guests will clearly be something
different, but I guess the interpretation of what invalid mappings mean
is up to the caller.

> Don't we
> need something that is non-null to preserve the table refcounting?

Sure, but do we care? If the table entry gets zeroed we're then
basically using an 'invalid block' mapping to annotate the entire block
range with '0', whatever that means. For guests it won't mean much, but
for the host that would mean sole ownership of the entire range.