[PATCH v5 0/8] KVM: arm64: permit MAP_SHARED mappings with MTE enabled

[Peter Collingbourne]

Hi,

This patch series allows VMMs to use shared mappings in MTE enabled
guests. The first five patches were taken from Catalin's tree [1] which
addressed some review feedback from when they were previously sent out
as v3 of this series. The first patch from Catalin's tree makes room
for an additional PG_arch_3 flag by making the newer PG_arch_* flags
arch-dependent. The next four patches are based on a series that
Catalin sent out prior to v3, whose cover letter [2] I quote from below:

> This series aims to fix the races between initialising the tags on a
> page and setting the PG_mte_tagged flag. Currently the flag is set
> either before or after that tag initialisation and this can lead to CoW
> copying stale tags. The first patch moves the flag setting after the
> tags have been initialised, solving the CoW issue. However, concurrent
> mprotect() on a shared mapping may (very rarely) lead to valid tags
> being zeroed.
>
> The second skips the sanitise_mte_tags() call in kvm_set_spte_gfn(),
> deferring it to user_mem_abort(). The outcome is that no
> sanitise_mte_tags() can be simplified to skip the pfn_to_online_page()
> check and only rely on VM_MTE_ALLOWED vma flag that can be checked in
> user_mem_abort().
>
> The third and fourth patches use PG_arch_3 as a lock for page tagging,
> based on Peter Collingbourne's idea of a two-bit lock.
>
> I think the first patch can be queued but the rest needs some in depth
> review and test. With this series (if correct) we could allos MAP_SHARED
> on KVM guest memory but this is to be discussed separately as there are
> some KVM ABI implications.

In this v5 I rebased Catalin's tree onto -next again. Please double check
my rebase, which resolved the conflict with commit a8e5e5146ad0 ("arm64:
mte: Avoid setting PG_mte_tagged if no tags cleared or restored").

I now have Reviewed-by for all patches except for the last one, which adds
the documentation. Thanks for the reviews so far, and please take a look!

I've tested it on QEMU as well as on MTE-capable hardware by booting a
Linux kernel and userspace under a crosvm with MTE support [3].

[1] git://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux devel/mte-pg-flags
[2] https://lore.kernel.org/all/20220705142619.4135905-1-catalin.marinas@arm.com/
[3] https://chromium-review.googlesource.com/c/crosvm/crosvm/+/3892141

Catalin Marinas (4):
  mm: Do not enable PG_arch_2 for all 64-bit architectures
  arm64: mte: Fix/clarify the PG_mte_tagged semantics
  KVM: arm64: Simplify the sanitise_mte_tags() logic
  arm64: mte: Lock a page for MTE tag initialisation

Peter Collingbourne (4):
  mm: Add PG_arch_3 page flag
  KVM: arm64: unify the tests for VMAs in memslots when MTE is enabled
  KVM: arm64: permit all VM_MTE_ALLOWED mappings with MTE enabled
  Documentation: document the ABI changes for KVM_CAP_ARM_MTE

 Documentation/virt/kvm/api.rst    |  5 ++-
 arch/arm64/Kconfig                |  1 +
 arch/arm64/include/asm/mte.h      | 65 ++++++++++++++++++++++++++++++-
 arch/arm64/include/asm/pgtable.h  |  4 +-
 arch/arm64/kernel/cpufeature.c    |  4 +-
 arch/arm64/kernel/elfcore.c       |  2 +-
 arch/arm64/kernel/hibernate.c     |  2 +-
 arch/arm64/kernel/mte.c           | 21 +++++-----
 arch/arm64/kvm/guest.c            | 18 +++++----
 arch/arm64/kvm/mmu.c              | 55 +++++++++++---------------
 arch/arm64/mm/copypage.c          |  7 +++-
 arch/arm64/mm/fault.c             |  4 +-
 arch/arm64/mm/mteswap.c           | 16 +++-----
 fs/proc/page.c                    |  3 +-
 include/linux/kernel-page-flags.h |  1 +
 include/linux/page-flags.h        |  3 +-
 include/trace/events/mmflags.h    |  9 +++--
 mm/Kconfig                        |  8 ++++
 mm/huge_memory.c                  |  3 +-
 19 files changed, 152 insertions(+), 79 deletions(-)

-- 
2.38.1.431.g37b22c650d-goog