Subject: Re: [PATCH 5/6] KVM: PPC: BOOK3S: Use hpte_update_in_progress to track invalid hpte during an hpte update

From: Paul Mackerras <[email-masked]>
Date: Wed, 2 Jul 2014 15:41:56 +1000

On Sun, Jun 29, 2014 at 04:47:34PM +0530, Aneesh Kumar K.V wrote:
> As per ISA, we first need to mark hpte invalid (V=0) before we update
> the hpte lower half bits. With virtual page class key protection mechanism we want
> to send any fault other than key fault to guest directly without
> searching the hash page table. But then we can get NO_HPTE fault while
> we are updating the hpte. To track that add a vm specific atomic
> variable that we check in the fault path to always send the fault
> to host.

[...]

> @@ -750,13 +751,15 @@ int kvmppc_book3s_hv_page_fault(struct kvm_run *run, struct kvm_vcpu *vcpu,
>  	r &= rcbits | ~(HPTE_R_R | HPTE_R_C);
>  
>  	if (be64_to_cpu(hptep[0]) & HPTE_V_VALID) {
> -		/* HPTE was previously valid, so we need to invalidate it */
> +		/*
> +		 * If we had mapped this hpte before, we now need to
> +		 * invalidate that.
> +		 */
>  		unlock_rmap(rmap);
> -		/* Always mark HPTE_V_ABSENT before invalidating */
> -		kvmppc_unmap_host_hpte(kvm, hptep);
>  		kvmppc_invalidate_hpte(kvm, hptep, index);
>  		/* don't lose previous R and C bits */
>  		r |= be64_to_cpu(hptep[1]) & (HPTE_R_R | HPTE_R_C);
> +		hpte_invalidated = true;

So now we're not setting the ABSENT bit before invalidating the HPTE.
That means that another guest vcpu could do an H_ENTER which could
think that this HPTE is free and use it for another unrelated guest
HPTE, which would be bad...

> @@ -1144,8 +1149,8 @@ static int kvm_test_clear_dirty_npages(struct kvm *kvm, unsigned long *rmapp)
>  				npages_dirty = n;
>  			eieio();
>  		}
> -		kvmppc_map_host_hpte(kvm, &v, &r);
> -		hptep[0] = cpu_to_be64(v & ~HPTE_V_HVLOCK);
> +		hptep[0] = cpu_to_be64(v & ~HPTE_V_LOCK);
> +		atomic_dec(&kvm->arch.hpte_update_in_progress);

Why are we using LOCK rather than HVLOCK now?  (And why didn't you
mention this change and its rationale in the patch description?)

Paul.