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From: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
To: Alexander Graf <agraf@suse.de>
Subject: Re: [PATCH v2] powernv: kvm: make _PAGE_NUMA take effect
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Date: Mon, 27 Jan 2014 15:58:45 +0530
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Cc: kvm-devel <kvm@vger.kernel.org>, kvm-ppc <kvm-ppc@vger.kernel.org>,
 Paul Mackerras <paulus@samba.org>, Liu Ping Fan <kernelfans@gmail.com>,
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Alexander Graf <agraf@suse.de> writes:

> 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?

As per the guest the entry is valid, so yes an hread should return a
valid entry. But in real hpte we would mark it not valid.

>
> I might need a crash course in the use of HPTE_V_ABSENT.

When guest tries to access the address, the host will handle the fault.

kvmppc_hpte_hv_fault should give more info

-aneesh