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

[Gleb Natapov]

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@xxxxxxxxxxxxxxxxxx>
> ---
>  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.
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