Re: [PATCH v2 01/27] KVM: Call kvm_arch_memslots_updated() before updating memslots

[Cornelia Huck <cohuck@redhat.com>]

On Tue,  5 Feb 2019 12:54:17 -0800
Sean Christopherson <sean.j.christopherson@intel.com> wrote:

> kvm_arch_memslots_updated() is at this point in time an x86-specific
> hook for handling MMIO generation wraparound.  x86 stashes 19 bits of
> the memslots generation number in its MMIO sptes in order to avoid
> full page fault walks for repeat faults on emulated MMIO addresses.
> Because only 19 bits are used, wrapping the MMIO generation number is
> possible, if unlikely.  kvm_arch_memslots_updated() alerts x86 that
> the generation has changed so that it can invalidate all MMIO sptes in
> case the effective MMIO generation has wrapped so as to avoid using a
> stale spte, e.g. a (very) old spte that was created with generation=0.
> 
> Given that the purpose of kvm_arch_memslots_updated() is to prevent
> consuming stale entries, it needs to be called before the new generation
> is propagated to memslots.  Invalidating the MMIO sptes after updating
> memslots means that there is a window where a vCPU could dereference
> the new memslots generation, e.g. 0, and incorrectly reuse an old MMIO
> spte that was created with (pre-wrap) generation=0.
> 
> Fixes: e59dbe09f8e6 ("KVM: Introduce kvm_arch_memslots_updated()")
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
> ---
>  arch/mips/include/asm/kvm_host.h    | 2 +-
>  arch/powerpc/include/asm/kvm_host.h | 2 +-
>  arch/s390/include/asm/kvm_host.h    | 2 +-
>  arch/x86/include/asm/kvm_host.h     | 2 +-
>  arch/x86/kvm/mmu.c                  | 4 ++--
>  arch/x86/kvm/x86.c                  | 4 ++--
>  include/linux/kvm_host.h            | 2 +-
>  virt/kvm/arm/mmu.c                  | 2 +-
>  virt/kvm/kvm_main.c                 | 7 +++++--
>  9 files changed, 15 insertions(+), 12 deletions(-)

Not an x86 person, but I think that makes sense.

Reviewed-by: Cornelia Huck <cohuck@redhat.com>