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Date: Mon, 8 Apr 2019 23:11:10 +0800
From: Zenghui Yu <yuzenghui@...wei.com>
To: Suzuki K Poulose <suzuki.poulose@....com>,
<linux-arm-kernel@...ts.infradead.org>
CC: <kvmarm@...ts.cs.columbia.edu>, <kvm@...r.kernel.org>,
<linux-kernel@...r.kernel.org>, <eric.auger@...hat.com>,
<marc.zyngier@....com>, <christoffer.dall@....com>,
<zhengxiang9@...wei.com>, <andrew.murray@....com>,
<wanghaibin.wang@...wei.com>
Subject: Re: [PATCH] kvm: arm: Skip stage2 huge mappings for unaligned ipa
backed by THP
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@...hat.com>
>>> Cc: Marc Zyngier <marc.zyngier@....com>
>>> Cc: Chirstoffer Dall <christoffer.dall@....com>
>>> Cc: Zenghui Yu <yuzenghui@...wei.com>
>>> Cc: Zheng Xiang <zhengxiang9@...wei.com>
>>> Tested-by: Eric Auger <eric.auger@....com>
>>> Signed-off-by: Suzuki K Poulose <suzuki.poulose@....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@...wei.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
>
> .
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