On 07/03/2013 04:39 PM, Xiao Guangrong wrote:
> 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@xxxxxxxxxxxxx>
>>> ---
>>> 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.
>
Actually, the double zapping can be avoided by moving kvm_mmu_invalidate_mmio_sptes to
the end of install_new_memslots().
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