• Paul Mackerras's avatar
    KVM: PPC: Book3S HV: Improve handling of local vs. global TLB invalidations · 1b400ba0
    Paul Mackerras authored
    When we change or remove a HPT (hashed page table) entry, we can do
    either a global TLB invalidation (tlbie) that works across the whole
    machine, or a local invalidation (tlbiel) that only affects this core.
    Currently we do local invalidations if the VM has only one vcpu or if
    the guest requests it with the H_LOCAL flag, though the guest Linux
    kernel currently doesn't ever use H_LOCAL.  Then, to cope with the
    possibility that vcpus moving around to different physical cores might
    expose stale TLB entries, there is some code in kvmppc_hv_entry to
    flush the whole TLB of entries for this VM if either this vcpu is now
    running on a different physical core from where it last ran, or if this
    physical core last ran a different vcpu.
    
    There are a number of problems on POWER7 with this as it stands:
    
    - The TLB invalidation is done per thread, whereas it only needs to be
      done per core, since the TLB is shared between the threads.
    - With the possibility of the host paging out guest pages, the use of
      H_LOCAL by an SMP guest is dangerous since the guest could possibly
      retain and use a stale TLB entry pointing to a page that had been
      removed from the guest.
    - The TLB invalidations that we do when a vcpu moves from one physical
      core to another are unnecessary in the case of an SMP guest that isn't
      using H_LOCAL.
    - The optimization of using local invalidations rather than global should
      apply to guests with one virtual core, not just one vcpu.
    
    (None of this applies on PPC970, since there we always have to
    invalidate the whole TLB when entering and leaving the guest, and we
    can't support paging out guest memory.)
    
    To fix these problems and simplify the code, we now maintain a simple
    cpumask of which cpus need to flush the TLB on entry to the guest.
    (This is indexed by cpu, though we only ever use the bits for thread
    0 of each core.)  Whenever we do a local TLB invalidation, we set the
    bits for every cpu except the bit for thread 0 of the core that we're
    currently running on.  Whenever we enter a guest, we test and clear the
    bit for our core, and flush the TLB if it was set.
    
    On initial startup of the VM, and when resetting the HPT, we set all the
    bits in the need_tlb_flush cpumask, since any core could potentially have
    stale TLB entries from the previous VM to use the same LPID, or the
    previous contents of the HPT.
    
    Then, we maintain a count of the number of online virtual cores, and use
    that when deciding whether to use a local invalidation rather than the
    number of online vcpus.  The code to make that decision is extracted out
    into a new function, global_invalidates().  For multi-core guests on
    POWER7 (i.e. when we are using mmu notifiers), we now never do local
    invalidations regardless of the H_LOCAL flag.
    Signed-off-by: default avatarPaul Mackerras <paulus@samba.org>
    Signed-off-by: default avatarAlexander Graf <agraf@suse.de>
    1b400ba0
book3s_hv.c 48.4 KB