Re: [PATCH v2] powernv: kvm: make _PAGE_NUMA take effect

[Paul Mackerras <paulus@samba.org>]

On Mon, Jan 27, 2014 at 10:11:40AM +0100, Alexander Graf wrote:
> 
> On 21.01.2014, at 10:42, Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com> wrote:
> 
> > Liu Ping Fan <kernelfans@gmail.com> writes:
> > 
> >> To make sure that on host, the pages marked with _PAGE_NUMA result in a fault
> >> when guest access them, we should force the checking when guest uses hypercall
> >> to setup hpte.
> >> 
> >> Signed-off-by: Liu Ping Fan <pingfank@linux.vnet.ibm.com>
> > 
> > Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
> > 
> > When we mark pte with _PAGE_NUMA we already call mmu_notifier_invalidate_range_start and
> > mmu_notifier_invalidate_range_end, which will mark existing guest hpte
> > entry as HPTE_V_ABSENT. Now we need to do that when we are inserting new
> > guest hpte entries. This patch does that. 
> 
> So what happens next? We insert a page into the HTAB without HPTE_V_VALID set, so the guest will fail to use it. If the guest does an H_READ on it it will suddenly turn to V_VALID though?
> 
> I might need a crash course in the use of HPTE_V_ABSENT.

HPTE_V_ABSENT means present from the point of view of the guest but
not present from the host's point of view, so yes H_READ turns
HPTE_V_ABSENT into HPTE_V_VALID.

Paul.