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Subject: Re: [PATCH] kvm: arm: Skip stage2 huge mappings for unaligned ipa
 backed by THP
To:     Suzuki K Poulose <suzuki.poulose@arm.com>,
        <linux-arm-kernel@lists.infradead.org>
CC:     <kvmarm@lists.cs.columbia.edu>, <kvm@vger.kernel.org>,
        <linux-kernel@vger.kernel.org>, <eric.auger@redhat.com>,
        <marc.zyngier@arm.com>, <christoffer.dall@arm.com>,
        <zhengxiang9@huawei.com>, <andrew.murray@arm.com>,
        <wanghaibin.wang@huawei.com>
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 <520e277c-f096-a84b-8405-636b19e4cc46@arm.com>
From:   Zenghui Yu <yuzenghui@huawei.com>
Message-ID: <8bf8e863-93f5-ab44-c0aa-1ad23f91a016@huawei.com>
Date:   Wed, 10 Apr 2019 10:20:30 +0800
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On 2019/4/9 22:59, Suzuki K Poulose wrote:
> Hi Zenghui
> 
> On 04/09/2019 09:05 AM, Zenghui Yu wrote:
>>
>>
>> On 2019/4/9 2:40, Suzuki K Poulose wrote:
>>> Hi Zenhui,
>>>
>>> On 04/08/2019 04:11 PM, Zenghui Yu wrote:
>>>> Hi Suzuki,
>>>>
>>>> Thanks for the reply.
>>>>
>>>
>>> ...
>>>
>>>>>> Hi Suzuki,
>>>>>>
>>>>>> Why not making use of fault_supports_stage2_huge_mapping()?  Let 
>>>>>> it do
>>>>>> some checks for us.
>>>>>>
>>>>>> fault_supports_stage2_huge_mapping() was intended to do a *two-step*
>>>>>> check to tell us that can we create stage2 huge block mappings, 
>>>>>> and this
>>>>>> check is both for hugetlbfs and THP.  With commit 
>>>>>> a80868f398554842b14,
>>>>>> we pass PAGE_SIZE as "map_size" for normal size pages (which 
>>>>>> turned out
>>>>>> to be almost meaningless), and unfortunately the THP check no longer
>>>>>> works.
>>>>>
>>>>> Thats correct.
>>>>>
>>>>>>
>>>>>> So we want to rework *THP* check process.  Your patch fixes the first
>>>>>> checking-step, but the second is still missed, am I wrong?
>>>>>
>>>>> It fixes the step explicitly for the THP by making sure that the 
>>>>> GPA and
>>>>> the HVA are aligned to the map size.
>>>>
>>>> Yes, I understand how your patch had fixed the issue.  But what I'm
>>>> really concerned about here is the *second* checking-step in
>>>> fault_supports_stage2_huge_mapping().
>>>>
>>>> We have to check if we are mapping a non-block aligned or non-block
>>>> sized memslot, if so, we can not create block mappings for the 
>>>> beginning
>>>> and end of this memslot.  This is what the second part of
>>>> fault_supports_stage2_huge_mapping() had done.
>>>>
>>>> I haven't seen this checking-step in your patch, did I miss something?
>>>>
>>>
>>> I see.
>>>
>>>>> I don't think this calls for a VM_BUG_ON(). It is simply a case where
>>>>> the GPA is not aligned to HVA, but for normal VMA that could be 
>>>>> made THP.
>>>>>
>>>>> We had this VM_BUG_ON(), which would have never hit because we would
>>>>> have set force_pte if they were not aligned.
>>>>
>>>> Yes, I agree.
>>>>
>>>>>>> +        /* Skip memslots with unaligned IPA and user address */
>>>>>>> +        if ((gfn & mask) != (pfn & mask))
>>>>>>> +            return false;
>>>>>>>           if (pfn & mask) {
>>>>>>>               *ipap &= PMD_MASK;
>>>>>>>               kvm_release_pfn_clean(pfn);
>>>>>>>
>>>>>>
>>>>>> ---8>---
>>>>>>
>>>>>> Rework fault_supports_stage2_huge_mapping(), let it check THP again.
>>>>>>
>>>>>> Signed-off-by: Zenghui Yu <yuzenghui@huawei.com>
>>>>>> ---
>>>>>>   virt/kvm/arm/mmu.c | 11 ++++++++++-
>>>>>>   1 file changed, 10 insertions(+), 1 deletion(-)
>>>>>>
>>>>>> diff --git a/virt/kvm/arm/mmu.c b/virt/kvm/arm/mmu.c
>>>>>> index 27c9583..5e1b258 100644
>>>>>> --- a/virt/kvm/arm/mmu.c
>>>>>> +++ b/virt/kvm/arm/mmu.c
>>>>>> @@ -1632,6 +1632,15 @@ static bool 
>>>>>> fault_supports_stage2_huge_mapping(struct kvm_memory_slot *memslot,
>>>>>>       uaddr_end = uaddr_start + size;
>>>>>>
>>>>>>       /*
>>>>>> +     * If the memslot is _not_ backed by hugetlbfs, then check if it
>>>>>> +     * can be backed by transparent hugepages.
>>>>>> +     *
>>>>>> +     * Currently only PMD_SIZE THPs are supported, revisit it later.
>>>>>> +     */
>>>>>> +    if (map_size == PAGE_SIZE)
>>>>>> +        map_size = PMD_SIZE;
>>>>>> +
>>>>>
>>>>> This looks hackish. What is we support PUD_SIZE huge page in the 
>>>>> future
>>>>> ?
>>>>
>>>> Yes, this might make the code a little difficult to understand. But by
>>>> doing so, we follow the same logic before commit a80868f398554842b14,
>>>> that said, we do the two-step checking for normal size pages in
>>>> fault_supports_stage2_huge_mapping(), to decide if we can create THP
>>>> mappings for these pages.
>>>>
>>>> As for PUD_SIZE THPs, to be honest, I have no idea now :(
>>>
>>> How about the following diff ?
>>>
>>> diff --git a/virt/kvm/arm/mmu.c b/virt/kvm/arm/mmu.c
>>> index 97b5417..98e5cec 100644
>>> --- a/virt/kvm/arm/mmu.c
>>> +++ b/virt/kvm/arm/mmu.c
>>> @@ -1791,7 +1791,8 @@ static int user_mem_abort(struct kvm_vcpu 
>>> *vcpu, phys_addr_t fault_ipa,
>>>            * currently supported. This code will need to be
>>>            * updated to support other THP sizes.
>>>            */
>>> -        if (transparent_hugepage_adjust(&pfn, &fault_ipa))
>>> +        if (fault_supports_stage2_huge_mappings(memslot, hva, 
>>> PMD_SIZE) &&
>>> +            transparent_hugepage_adjust(&pfn, &fault_ipa))
>>>               vma_pagesize = PMD_SIZE;
>>>       }
>>
>> I think this is good enough for the issue.
>>
>> (One minor concern: With this change, it seems that we no longer need
>> "force_pte" and can just use "logging_active" instead. But this is not
>> much related to what we're fixing.)
> 
> I would still leave the force_pte there to avoid checking for a THP case
> in a situation where we forced to PTE level mapping on a hugepage backed
> VMA. It would serve to avoid another check.

Hi Suzuki,

Yes, I agree, thanks.


zenghui