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

[Gleb Natapov <gleb@redhat.com>]

On Wed, Jul 03, 2013 at 04:50:36PM +0800, Xiao Guangrong wrote:
> 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@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.
> > 
> 
> Actually, the double zapping can be avoided by moving kvm_mmu_invalidate_mmio_sptes to
> the end of install_new_memslots().
> 
Exactly. Why should we hide it in obscure functions?

--
			Gleb.