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

[Alexander Graf]

On 27.01.2014, at 11:28, Aneesh Kumar K.V <aneesh.kumar at linux.vnet.ibm.com> wrote:

> Alexander Graf <agraf at suse.de> writes:
> 
>> On 21.01.2014, at 10:42, Aneesh Kumar K.V <aneesh.kumar at linux.vnet.ibm.com> wrote:
>> 
>>> Liu Ping Fan <kernelfans at 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 at linux.vnet.ibm.com>
>>> 
>>> Reviewed-by: Aneesh Kumar K.V <aneesh.kumar at 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.

Ah, yes.

> 
>> 
>> 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

Thanks for the pointer. So we fault it in lazily. Is there any particular reason we can't do that on h_enter already? After all this just means an additional roundtrip because the guest is pretty likely to use the page it just entered, no?


Alex