1. 21 Oct, 2019 1 commit
    • Christoffer Dall's avatar
      KVM: arm/arm64: Allow reporting non-ISV data aborts to userspace · c726200d
      Christoffer Dall authored
      For a long time, if a guest accessed memory outside of a memslot using
      any of the load/store instructions in the architecture which doesn't
      supply decoding information in the ESR_EL2 (the ISV bit is not set), the
      kernel would print the following message and terminate the VM as a
      result of returning -ENOSYS to userspace:
      
        load/store instruction decoding not implemented
      
      The reason behind this message is that KVM assumes that all accesses
      outside a memslot is an MMIO access which should be handled by
      userspace, and we originally expected to eventually implement some sort
      of decoding of load/store instructions where the ISV bit was not set.
      
      However, it turns out that many of the instructions which don't provide
      decoding information on abort are not safe to use for MMIO accesses, and
      the remaining few that would potentially make sense to use on MMIO
      accesses, such as those with register writeback, are not used in
      practice.  It also turns out that fetching an instruction from guest
      memory can be a pretty horrible affair, involving stopping all CPUs on
      SMP systems, handling multiple corner cases of address translation in
      software, and more.  It doesn't appear likely that we'll ever implement
      this in the kernel.
      
      What is much more common is that a user has misconfigured his/her guest
      and is actually not accessing an MMIO region, but just hitting some
      random hole in the IPA space.  In this scenario, the error message above
      is almost misleading and has led to a great deal of confusion over the
      years.
      
      It is, nevertheless, ABI to userspace, and we therefore need to
      introduce a new capability that userspace explicitly enables to change
      behavior.
      
      This patch introduces KVM_CAP_ARM_NISV_TO_USER (NISV meaning Non-ISV)
      which does exactly that, and introduces a new exit reason to report the
      event to userspace.  User space can then emulate an exception to the
      guest, restart the guest, suspend the guest, or take any other
      appropriate action as per the policy of the running system.
      Reported-by: default avatarHeinrich Schuchardt <xypron.glpk@gmx.de>
      Signed-off-by: default avatarChristoffer Dall <christoffer.dall@arm.com>
      Reviewed-by: default avatarAlexander Graf <graf@amazon.com>
      Signed-off-by: default avatarMarc Zyngier <maz@kernel.org>
      c726200d
  2. 13 Oct, 2019 16 commits
  3. 12 Oct, 2019 23 commits