Re: [PATCH v2 6/6] KVM: MMU: init kvm generation close to mmio wrap-around value

Gleb Natapov
2013-04-24T12:59:29Z

On Mon, Apr 01, 2013 at 05:56:49PM +0800, Xiao Guangrong wrote:
> Then it has chance to trigger mmio generation number wrap-around
>
> Signed-off-by: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
> ---
> arch/x86/include/asm/kvm_host.h | 1 +
> arch/x86/kvm/mmu.c | 8 ++++++++
> virt/kvm/kvm_main.c | 6 ++++++
> 3 files changed, 15 insertions(+), 0 deletions(-)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 6c1e642..4e1f7cb 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -767,6 +767,7 @@ void kvm_mmu_write_protect_pt_masked(struct kvm *kvm,
> struct kvm_memory_slot *slot,
> gfn_t gfn_offset, unsigned long mask);
> void kvm_mmu_zap_all(struct kvm *kvm);
> +void kvm_arch_init_generation(struct kvm *kvm);
> void kvm_mmu_invalid_mmio_sptes(struct kvm *kvm);
> unsigned int kvm_mmu_calculate_mmu_pages(struct kvm *kvm);
> void kvm_mmu_change_mmu_pages(struct kvm *kvm, unsigned int kvm_nr_mmu_pages);
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index d314e21..dcc059c 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -4279,6 +4279,14 @@ restart:
> spin_unlock(&kvm->mmu_lock);
> }
>
> +void kvm_arch_init_generation(struct kvm *kvm)
> +{
> + mutex_lock(&kvm->slots_lock);
> + /* It is easier to trigger mmio generation-number wrap-around. */
> + kvm_memslots(kvm)->generation = MMIO_MAX_GEN - 13;
kvm_memslots(kvm)->generation should never overflow since
(read|write)_cached mechanism does not handle it. Initialising it to
anything but 0 makes overflow more likely.

You can hide mmio overflow trick in kvm_current_mmio_generation():

static unsigned int kvm_current_mmio_generation(struct kvm *kvm)
{
return (kvm_memslots(kvm)->generation + MMIO_MAX_GEN - 13) & MMIO_GEN_MASK;
}

--
Gleb.