[RFC PATCH] KVM: arm64: Add prejudgement for relaxing permissions only case in stage2 translation fault handler

[wangyanan (Y)]

On 2020/12/11 17:49, Marc Zyngier wrote:
> Hi Yanan,
>
> On 2020-12-11 08:01, Yanan Wang wrote:
>> In dirty-logging, or dirty-logging-stopped time, even normal running
>> time of a guest configed with huge mappings and numbers of vCPUs,
>> translation faults by different vCPUs on the same GPA could occur
>> successively almost at the same time. There are two reasons for it.
>>
>> (1) If there are some vCPUs accessing the same GPA at the same time
>> and the leaf PTE is not set yet, then they will all cause translation
>> faults and the first vCPU holding mmu_lock will set valid leaf PTE,
>> and the others will later choose to update the leaf PTE or not.
>>
>> (2) When changing a leaf entry or a table entry with break-before-make,
>> if there are some vCPUs accessing the same GPA just catch the moment
>> when the target PTE is set invalid in a BBM procedure coincidentally,
>> they will all cause translation faults and will later choose to update
>> the leaf PTE or not.
>>
>> The worst case can be like this: some vCPUs cause translation faults
>> on the same GPA with different prots, they will fight each other by
>> changing back access permissions of the PTE with break-before-make.
>> And the BBM-invalid moment might trigger more unnecessary translation
>> faults. As a result, some useless small loops will occur, which could
>> lead to vCPU stuck.
>>
>> To avoid unnecessary update and small loops, add prejudgement in the
>> translation fault handler: Skip updating the valid leaf PTE if we are
>> trying to recreate exactly the same mapping or to reduce access
>> permissions only(such as RW-->RO). And update the valid leaf PTE without
>> break-before-make if we are trying to add more permissions only.
>
> I'm a bit perplexed with this: why are you skipping the update if the
> permissions need to be reduced? Even more, how can we reduce the
> permissions from a vCPU fault? I can't really think of a scenario where
> that happens.
>
> Or are you describing a case where two vcpus fault simultaneously with
> conflicting permissions:
>
> - Both vcpus fault on translation fault
> - vcpu A wants W access
> - vpcu B wants R access
>
> and 'A' gets in first, set the permissions to RW (because R is
> implicitly added to W), followed by 'B' which downgrades it to RO?
>
> If that's what you are describing, then I agree we could do better.
Yes, this is exactly what I want to describe.
>
>>
>> Signed-off-by: Yanan Wang <wangyanan55 at huawei.com>
>> ---
>>  arch/arm64/kvm/hyp/pgtable.c | 73 +++++++++++++++++++++++++-----------
>>  1 file changed, 52 insertions(+), 21 deletions(-)
>>
>> diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
>> index 23a01dfcb27a..f8b3248cef1c 100644
>> --- a/arch/arm64/kvm/hyp/pgtable.c
>> +++ b/arch/arm64/kvm/hyp/pgtable.c
>> @@ -45,6 +45,8 @@
>>
>>  #define KVM_PTE_LEAF_ATTR_HI_S2_XN    BIT(54)
>>
>> +#define KVM_PTE_LEAF_ATTR_PERMS    (GENMASK(7, 6) | BIT(54))
>> +
>>  struct kvm_pgtable_walk_data {
>>      struct kvm_pgtable        *pgt;
>>      struct kvm_pgtable_walker    *walker;
>> @@ -170,10 +172,9 @@ static void kvm_set_table_pte(kvm_pte_t *ptep,
>> kvm_pte_t *childp)
>>      smp_store_release(ptep, pte);
>>  }
>>
>> -static bool kvm_set_valid_leaf_pte(kvm_pte_t *ptep, u64 pa, 
>> kvm_pte_t attr,
>> -                   u32 level)
>> +static kvm_pte_t kvm_init_valid_leaf_pte(u64 pa, kvm_pte_t attr, u32 
>> level)
>>  {
>> -    kvm_pte_t old = *ptep, pte = kvm_phys_to_pte(pa);
>> +    kvm_pte_t pte = kvm_phys_to_pte(pa);
>>      u64 type = (level == KVM_PGTABLE_MAX_LEVELS - 1) ? 
>> KVM_PTE_TYPE_PAGE :
>>                                 KVM_PTE_TYPE_BLOCK;
>>
>> @@ -181,12 +182,7 @@ static bool kvm_set_valid_leaf_pte(kvm_pte_t
>> *ptep, u64 pa, kvm_pte_t attr,
>>      pte |= FIELD_PREP(KVM_PTE_TYPE, type);
>>      pte |= KVM_PTE_VALID;
>>
>> -    /* Tolerate KVM recreating the exact same mapping. */
>> -    if (kvm_pte_valid(old))
>> -        return old == pte;
>> -
>> -    smp_store_release(ptep, pte);
>> -    return true;
>> +    return pte;
>>  }
>>
>>  static int kvm_pgtable_visitor_cb(struct kvm_pgtable_walk_data 
>> *data, u64 addr,
>> @@ -341,12 +337,17 @@ static int hyp_map_set_prot_attr(enum
>> kvm_pgtable_prot prot,
>>  static bool hyp_map_walker_try_leaf(u64 addr, u64 end, u32 level,
>>                      kvm_pte_t *ptep, struct hyp_map_data *data)
>>  {
>> +    kvm_pte_t new, old = *ptep;
>>      u64 granule = kvm_granule_size(level), phys = data->phys;
>>
>>      if (!kvm_block_mapping_supported(addr, end, phys, level))
>>          return false;
>>
>> -    WARN_ON(!kvm_set_valid_leaf_pte(ptep, phys, data->attr, level));
>> +    /* Tolerate KVM recreating the exact same mapping. */
>> +    new = kvm_init_valid_leaf_pte(phys, data->attr, level);
>> +    if (old != new && !WARN_ON(kvm_pte_valid(old)))
>> +        smp_store_release(ptep, new);
>> +
>>      data->phys += granule;
>>      return true;
>>  }
>> @@ -461,25 +462,56 @@ static int stage2_map_set_prot_attr(enum
>> kvm_pgtable_prot prot,
>>      return 0;
>>  }
>>
>> +static bool stage2_set_valid_leaf_pte_pre(u64 addr, u32 level,
>> +                      kvm_pte_t *ptep, kvm_pte_t new,
>> +                      struct stage2_map_data *data)
>> +{
>> +    kvm_pte_t old = *ptep, old_attr, new_attr;
>> +
>> +    if ((old ^ new) & (~KVM_PTE_LEAF_ATTR_PERMS))
>> +        return false;
>> +
>> +    /*
>> +     * Skip updating if we are trying to recreate exactly the same 
>> mapping
>> +     * or to reduce the access permissions only. And update the 
>> valid leaf
>> +     * PTE without break-before-make if we are trying to add more 
>> access
>> +     * permissions only.
>> +     */
>> +    old_attr = (old & KVM_PTE_LEAF_ATTR_PERMS) ^ 
>> KVM_PTE_LEAF_ATTR_HI_S2_XN;
>> +    new_attr = (new & KVM_PTE_LEAF_ATTR_PERMS) ^ 
>> KVM_PTE_LEAF_ATTR_HI_S2_XN;
>> +    if (new_attr <= old_attr)
>> +        return true;
>> +
>> +    WRITE_ONCE(*ptep, new);
>> +    kvm_call_hyp(__kvm_tlb_flush_vmid_ipa, data->mmu, addr, level);
>
> I think what bothers me the most here is that we are turning a mapping 
> into
> a permission update, which makes the code really hard to read, and mixes
> two things that were so far separate.
>
> I wonder whether we should instead abort the update and simply take 
> the fault
> again, if we ever need to do it.
>
> Thanks,
>
>         M.