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

[Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>]

On 04/24/2013 08:59 PM, Gleb Natapov wrote:
> 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;
> }

Very smart idea. Thanks you, Gleb!