[PATCH] powerpc: kvm: make the setup of hpte under the protection of KVMPPC_RMAP_LOCK_BIT

[Benjamin Herrenschmidt]

On Mon, 2014-07-28 at 14:09 +0800, Liu Ping Fan wrote:
> In current code, the setup of hpte is under the risk of race with
> mmu_notifier_invalidate, i.e we may setup a hpte with a invalid pfn.
> Resolve this issue by sync the two actions by KVMPPC_RMAP_LOCK_BIT.

Please describe the race you think you see. I'm quite sure both Paul and
I went over that code and somewhat convinced ourselves that it was ok
but it's possible that we were both wrong :-)

Cheers,
Ben.

> Signed-off-by: Liu Ping Fan <pingfank at linux.vnet.ibm.com>
> ---
>  arch/powerpc/kvm/book3s_64_mmu_hv.c | 15 ++++++++++-----
>  1 file changed, 10 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/powerpc/kvm/book3s_64_mmu_hv.c b/arch/powerpc/kvm/book3s_64_mmu_hv.c
> index 8056107..e6dcff4 100644
> --- a/arch/powerpc/kvm/book3s_64_mmu_hv.c
> +++ b/arch/powerpc/kvm/book3s_64_mmu_hv.c
> @@ -754,19 +754,24 @@ int kvmppc_book3s_hv_page_fault(struct kvm_run *run, struct kvm_vcpu *vcpu,
>  
>  	if (hptep[0] & HPTE_V_VALID) {
>  		/* HPTE was previously valid, so we need to invalidate it */
> -		unlock_rmap(rmap);
>  		hptep[0] |= HPTE_V_ABSENT;
>  		kvmppc_invalidate_hpte(kvm, hptep, index);
>  		/* don't lose previous R and C bits */
>  		r |= hptep[1] & (HPTE_R_R | HPTE_R_C);
> +
> +		hptep[1] = r;
> +		eieio();
> +		hptep[0] = hpte[0];
> +		asm volatile("ptesync" : : : "memory");
> +		unlock_rmap(rmap);
>  	} else {
> +		hptep[1] = r;
> +		eieio();
> +		hptep[0] = hpte[0];
> +		asm volatile("ptesync" : : : "memory");
>  		kvmppc_add_revmap_chain(kvm, rev, rmap, index, 0);
>  	}
>  
> -	hptep[1] = r;
> -	eieio();
> -	hptep[0] = hpte[0];
> -	asm volatile("ptesync" : : : "memory");
>  	preempt_enable();
>  	if (page && hpte_is_writable(r))
>  		SetPageDirty(page);