Subject: Re: [PATCH] KVM: PPC: BOOK3S: HV: Use base page size when comparing against slb value

From: "Aneesh Kumar K.V" <[email-masked]>
Date: Fri, 13 Jun 2014 22:13:24 +0530

Alexander Graf <[email-masked]> writes:

> On 13.06.14 16:28, Aneesh Kumar K.V wrote:
>> Alexander Graf <[email-masked]> writes:
>>
>>> On 13.06.14 09:23, Aneesh Kumar K.V wrote:
>>>> With guest supporting Multiple page size per segment (MPSS),
>>>> hpte_page_size returns actual page size used. Add a new function to
>>>> return base page size and use that to compare against the the page size
>>>> calculated from SLB
>>> Why? What does this fix? Is this a bug fix, an enhancement? Don't
>>> describe only what you do, but also why you do it.
>>>
>>>
>> This could result in page fault failures (unhandled page fault) because
>> even though we have a valid hpte entry mapping a 16MB page, since we
>> were comparing actual page size against page size calculated from SLB
>> bits kvmppc_hv_find_lock_hpte will fail and return -1. I did not observe
>> a failure in real and the bug was found during code audit. That could be
>> because with THP we have guest ram backed by hugetlbfs and we always
>> find the page in the host linux page table. The will result in do_h_enter always
>> inserting HPTE_V_VALID entry and hence we might not really end up calling
>> kvmppc_hv_find_lock_hpte.
>
> So why do we need to override to base page size for the VRMA region?

slb encoding should be derived based on base page size. 

> Also I think you want to change the comment above the line in 
> find_lock_hpte you're changing.
>

Will do that.

-aneesh