linux-kernel - Re: [PATCH v5 29/36] KVM: arm64: Use page-table to track page ownership

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Date:   Mon, 15 Mar 2021 17:01:33 +0000
From:   Will Deacon <will@...nel.org>
To:     Quentin Perret <qperret@...gle.com>
Cc:     catalin.marinas@....com, maz@...nel.org, james.morse@....com,
        julien.thierry.kdev@...il.com, suzuki.poulose@....com,
        android-kvm@...gle.com, seanjc@...gle.com, mate.toth-pal@....com,
        linux-kernel@...r.kernel.org, robh+dt@...nel.org,
        linux-arm-kernel@...ts.infradead.org, kernel-team@...roid.com,
        kvmarm@...ts.cs.columbia.edu, tabba@...gle.com, ardb@...nel.org,
        mark.rutland@....com, dbrazdil@...gle.com
Subject: Re: [PATCH v5 29/36] KVM: arm64: Use page-table to track page
 ownership

On Mon, Mar 15, 2021 at 04:53:18PM +0000, Quentin Perret wrote:
> On Monday 15 Mar 2021 at 16:36:19 (+0000), Will Deacon wrote:
> > On Mon, Mar 15, 2021 at 02:35:29PM +0000, Quentin Perret wrote:
> > > As the host stage 2 will be identity mapped, all the .hyp memory regions
> > > and/or memory pages donated to protected guestis will have to marked
> > > invalid in the host stage 2 page-table. At the same time, the hypervisor
> > > will need a way to track the ownership of each physical page to ensure
> > > memory sharing or donation between entities (host, guests, hypervisor) is
> > > legal.
> > > 
> > > In order to enable this tracking at EL2, let's use the host stage 2
> > > page-table itself. The idea is to use the top bits of invalid mappings
> > > to store the unique identifier of the page owner. The page-table owner
> > > (the host) gets identifier 0 such that, at boot time, it owns the entire
> > > IPA space as the pgd starts zeroed.
> > > 
> > > Provide kvm_pgtable_stage2_set_owner() which allows to modify the
> > > ownership of pages in the host stage 2. It re-uses most of the map()
> > > logic, but ends up creating invalid mappings instead. This impacts
> > > how we do refcount as we now need to count invalid mappings when they
> > > are used for ownership tracking.
> > > 
> > > Signed-off-by: Quentin Perret <qperret@...gle.com>
> > > ---
> > >  arch/arm64/include/asm/kvm_pgtable.h |  21 +++++
> > >  arch/arm64/kvm/hyp/pgtable.c         | 127 ++++++++++++++++++++++-----
> > >  2 files changed, 124 insertions(+), 24 deletions(-)
> > > 
> > > diff --git a/arch/arm64/include/asm/kvm_pgtable.h b/arch/arm64/include/asm/kvm_pgtable.h
> > > index 4ae19247837b..683e96abdc24 100644
> > > --- a/arch/arm64/include/asm/kvm_pgtable.h
> > > +++ b/arch/arm64/include/asm/kvm_pgtable.h
> > > @@ -238,6 +238,27 @@ int kvm_pgtable_stage2_map(struct kvm_pgtable *pgt, u64 addr, u64 size,
> > >  			   u64 phys, enum kvm_pgtable_prot prot,
> > >  			   void *mc);
> > >  
> > > +/**
> > > + * kvm_pgtable_stage2_set_owner() - Annotate invalid mappings with metadata
> > > + *				    encoding the ownership of a page in the
> > > + *				    IPA space.
> > 
> > The function does more than this, though, as it will also go ahead and unmap
> > existing valid mappings which I think should be mentioned here, no?
> 
> Right, I see why you mean. How about:
> 
> 'Unmap and annotate pages in the IPA space to track ownership'

I think I'd go with:

'Unmap pages and annotate the invalid mappings with ownership metadata for
 the unmapped IPA range.'

as it's the page-table which is annotated, not the actual pages (which could
potentially be mapped by other page-tables).

Will