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

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Date:   Mon, 8 Apr 2019 11:35:30 +0100
From:   Suzuki K Poulose <suzuki.poulose@....com>
To:     yuzenghui@...wei.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

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.

> 
> 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.

>> +        /* 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
?

> +    /*
>        * 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