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    SubjectRe: [PATCH v5 29/36] KVM: arm64: Use page-table to track page ownership
    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?

    > +int kvm_pgtable_stage2_set_owner(struct kvm_pgtable *pgt, u64 addr, u64 size,
    > + void *mc, u8 owner_id)
    > +{
    > + int ret;
    > + struct stage2_map_data map_data = {
    > + .phys = KVM_PHYS_INVALID,
    > + .mmu = pgt->mmu,
    > + .memcache = mc,
    > + .mm_ops = pgt->mm_ops,
    > + .owner_id = owner_id,
    > + };
    > + struct kvm_pgtable_walker walker = {
    > + .cb = stage2_map_walker,
    > + .flags = KVM_PGTABLE_WALK_TABLE_PRE |
    > + KVM_PGTABLE_WALK_LEAF |
    > + KVM_PGTABLE_WALK_TABLE_POST,
    > + .arg = &map_data,
    > + };
    > +
    > + if (owner_id > KVM_MAX_OWNER_ID)
    > + return -EINVAL;
    > +
    > + ret = kvm_pgtable_walk(pgt, addr, size, &walker);
    > + dsb(ishst);

    Why is the DSB needed here? afaict, we only ever unmap a valid entry (which
    will have a DSB as part of the TLBI sequence) or we update the owner for an
    existing invalid entry, in which case the walker doesn't care.

    Will

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