Re: [PATCH] KVM: x86: Avoid zapping mmio sptes twice for generation wraparound

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

On 07/03/2013 04:28 PM, Paolo Bonzini wrote:
> Il 03/07/2013 10:18, Takuya Yoshikawa ha scritto:
>> Since kvm_arch_prepare_memory_region() is called right after installing
>> the slot marked invalid, wraparound checking should be there to avoid
>> zapping mmio sptes when mmio generation is still MMIO_MAX_GEN - 1.
>>
>> Signed-off-by: Takuya Yoshikawa <yoshikawa_takuya_b1@lab.ntt.co.jp>
>> ---
>>  This seems to be the simplest solution for fixing the off-by-one issue
>>  we discussed before.
>>
>>  arch/x86/kvm/mmu.c |    5 +----
>>  arch/x86/kvm/x86.c |    7 +++++++
>>  2 files changed, 8 insertions(+), 4 deletions(-)
>>
>> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
>> index 0d094da..bf7af1e 100644
>> --- a/arch/x86/kvm/mmu.c
>> +++ b/arch/x86/kvm/mmu.c
>> @@ -4383,11 +4383,8 @@ void kvm_mmu_invalidate_mmio_sptes(struct kvm *kvm)
>>  	/*
>>  	 * The very rare case: if the generation-number is round,
>>  	 * zap all shadow pages.
>> -	 *
>> -	 * The max value is MMIO_MAX_GEN - 1 since it is not called
>> -	 * when mark memslot invalid.
>>  	 */
>> -	if (unlikely(kvm_current_mmio_generation(kvm) >= (MMIO_MAX_GEN - 1))) {
>> +	if (unlikely(kvm_current_mmio_generation(kvm) >= MMIO_MAX_GEN)) {
>>  		printk_ratelimited(KERN_INFO "kvm: zapping shadow pages for mmio generation wraparound\n");
>>  		kvm_mmu_invalidate_zap_all_pages(kvm);
>>  	}
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index 7d71c0f..9ddd4ff 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -7046,6 +7046,13 @@ int kvm_arch_prepare_memory_region(struct kvm *kvm,
>>  		memslot->userspace_addr = userspace_addr;
>>  	}
>>  
>> +	/*
>> +	 * In these cases, slots->generation has been increased for marking the
>> +	 * slot invalid, so we need wraparound checking here.
>> +	 */
>> +	if ((change == KVM_MR_DELETE) || (change == KVM_MR_MOVE))
>> +		kvm_mmu_invalidate_mmio_sptes(kvm);
>> +
>>  	return 0;
>>  }
>>  
>>
> 
> Applied, thanks.

Please wait a while. I can not understand it very clearly.

This conditional check will cause caching a overflow value into mmio spte.
The simple case is that kvm adds new slots for many times, the mmio-gen is easily
more than MMIO_MAX_GEN.