[PATCH v2 00/15] KVM: arm64: pkvm: Implement unshare hypercall

Quentin Perret
2021-10-19T12:13:12Z

Hi all,

This is v2 of the series previously posted here:

https://lore.kernel.org/kvmarm/20211013155831.943476-1-qperret@google.com/

This series implements an unshare hypercall at EL2 in nVHE protected
mode, and makes use of it to unmmap guest-specific data-structures from
EL2 stage-1 during guest tear-down. Crucially, the implementation of the
share and unshare routines use page refcounts in the host kernel to
avoid accidentally unmapping data-structures that overlap a common page.

This series has two main benefits. Firstly it allows EL2 to track the
state of shared pages cleanly, as they can now transition from SHARED
back to OWNED. This will simplify permission checks once e.g. pkvm
implements a donation hcall to provide memory to protected guests, as
there should then be no reason for the host to donate a page that is
currently marked shared. And secondly, it avoids having dangling
mappings in the hypervisor's stage-1, which should be a good idea from
a security perspective as the hypervisor is obviously running with
elevated privileges. And perhaps worth noting is that this also
refactors the EL2 page-tracking checks in a more scalable way, which
should allow to implement other memory transitions (host donating memory
to a guest, a guest sharing back with the host, ...) much more easily in
the future.

Changes since v2:

- moved the refcounting of pages shared more than once to the host in
order to simplify and optimize the hyp code;

- synchronized lifetime of the vcpu and its parent task struct using
get_task_struct() / put_task_struct();

- rebased on kvmarm/next

- rebased on Marc's v2 refactoring of the first vcpu run:
https://lore.kernel.org/kvmarm/20211018211158.3050779-1-maz@kernel.org

- small improvements/refactoring throughout;

This has been lightly tested on Qemu, by spawning and powering off a
guest 50 times. You can find a branch with everything applied here:

https://android-kvm.googlesource.com/linux qperret/hyp-unshare-v2

Thanks!
Quentin

Quentin Perret (7):
KVM: arm64: Check if running in VHE from kvm_host_owns_hyp_mappings()
KVM: arm64: Provide {get,put}_page() stubs for early hyp allocator
KVM: arm64: Refcount hyp stage-1 pgtable pages
KVM: arm64: Fixup hyp stage-1 refcount
KVM: arm64: Introduce kvm_share_hyp()
KVM: arm64: pkvm: Refcount the pages shared with EL2
KVM: arm64: pkvm: Unshare guest structs during teardown

Will Deacon (8):
KVM: arm64: Hook up ->page_count() for hypervisor stage-1 page-table
KVM: arm64: Implement kvm_pgtable_hyp_unmap() at EL2
KVM: arm64: Extend pkvm_page_state enumeration to handle absent pages
KVM: arm64: Introduce wrappers for host and hyp spin lock accessors
KVM: arm64: Implement do_share() helper for sharing memory
KVM: arm64: Implement __pkvm_host_share_hyp() using do_share()
KVM: arm64: Implement do_unshare() helper for unsharing memory
KVM: arm64: Expose unshare hypercall to the host

arch/arm64/include/asm/kvm_asm.h | 1 +
arch/arm64/include/asm/kvm_host.h | 2 +
arch/arm64/include/asm/kvm_mmu.h | 2 +
arch/arm64/include/asm/kvm_pgtable.h | 21 +
arch/arm64/kvm/arm.c | 6 +-
arch/arm64/kvm/fpsimd.c | 33 +-
arch/arm64/kvm/hyp/include/nvhe/mem_protect.h | 6 +
arch/arm64/kvm/hyp/nvhe/early_alloc.c | 5 +
arch/arm64/kvm/hyp/nvhe/hyp-main.c | 8 +
arch/arm64/kvm/hyp/nvhe/mem_protect.c | 500 +++++++++++++++---
arch/arm64/kvm/hyp/nvhe/setup.c | 32 +-
arch/arm64/kvm/hyp/pgtable.c | 80 ++-
arch/arm64/kvm/mmu.c | 132 ++++-
arch/arm64/kvm/reset.c | 10 +-
14 files changed, 733 insertions(+), 105 deletions(-)

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2.33.0.1079.g6e70778dc9-goog