Re: [PATCH] kvm: arm: Skip stage2 huge mappings for unaligned ipa backed by THP

[Zenghui Yu <yuzenghui@huawei.com>]

Hi Suzuki,

Thanks for the reply.

On 2019/4/8 18:35, Suzuki K Poulose wrote:
> On 04/08/2019 04:50 AM, Zenghui Yu wrote:
>>
>>
>> On 2019/4/2 19:06, Suzuki K Poulose wrote:
>>> With commit a80868f398554842b14, we no longer ensure that the
>>> THP page is properly aligned in the guest IPA. Skip the stage2
>>> huge mapping for unaligned IPA backed by transparent hugepages.
>>>
>>> Fixes: a80868f398554842b14 ("KVM: arm/arm64: Enforce PTE mappings at 
>>> stage2 when needed")
>>> Reported-by: Eric Auger <eric.auger@redhat.com>
>>> Cc: Marc Zyngier <marc.zyngier@arm.com>
>>> Cc: Chirstoffer Dall <christoffer.dall@arm.com>
>>> Cc: Zenghui Yu <yuzenghui@huawei.com>
>>> Cc: Zheng Xiang <zhengxiang9@huawei.com>
>>> Tested-by: Eric Auger <eric.auger@arm.com>
>>> Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
>>
>> 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?

>>
>> Can you please give a look at the below diff?
>>
>>
>> thanks,
>>
>> zenghui
>>
>>> ---
>>>   virt/kvm/arm/mmu.c | 4 +++-
>>>   1 file changed, 3 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/virt/kvm/arm/mmu.c b/virt/kvm/arm/mmu.c
>>> index 27c9583..4a22f5b 100644
>>> --- a/virt/kvm/arm/mmu.c
>>> +++ b/virt/kvm/arm/mmu.c
>>> @@ -1412,7 +1412,9 @@ static bool 
>>> transparent_hugepage_adjust(kvm_pfn_t *pfnp, phys_addr_t *ipap)
>>>            * page accordingly.
>>>            */
>>>           mask = PTRS_PER_PMD - 1;
>>> -        VM_BUG_ON((gfn & mask) != (pfn & mask));
>> Somehow, I'd prefer keeping the VM_BUG_ON() here, let it report some
>> potential issues in the future (of course I hope none:) )
> 
> 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 :(



thanks,

zenghui

>> +    /*
>>        * Pages belonging to memslots that don't have the same alignment
>>        * within a PMD/PUD for userspace and IPA cannot be mapped with 
>> stage-2
>>        * PMD/PUD entries, because we'll end up mapping the wrong pages.
>> @@ -1643,7 +1652,7 @@ static bool 
>> fault_supports_stage2_huge_mapping(struct kvm_memory_slot *memslot,
>>        *    |abcde|fgh  Stage-1 block  |    Stage-1 block tv|xyz|
>>        *    +-----+--------------------+--------------------+---+
>>        *
>> -     *    memslot->base_gfn << PAGE_SIZE:
>> +     *    memslot->base_gfn << PAGE_SHIFT:
> 
> That could be fixed.
> 
>>        *      +---+--------------------+--------------------+-----+
>>        *      |abc|def  Stage-2 block  |    Stage-2 block   |tvxyz|
>>        *      +---+--------------------+--------------------+-----+
> 
> 
> But personally I don't think this is the right way to fix it. And as
> mentioned, the VM_BUG_ON() is unnecessary and could easily be triggered
> by the user by using unaligned GPA/HVA. All we need to do is, use page
> mapping in such cases, which we do with my patch.
> 
> 
> Cheers
> Suzuki
> 
> .