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    SubjectRe: [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@google.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
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