Re: [PATCH 08/26] KVM: arm64: Use TTL hint in when invalidating stage-2 translations

James Morse
2020-05-12T12:04:40Z

Hi Andrew,

On 07/05/2020 16:13, Andrew Scull wrote:
>> @@ -176,7 +177,7 @@ static void clear_stage2_pud_entry(struct kvm_s2_mmu *mmu, pud_t *pud, phys_addr
>> pmd_t *pmd_table __maybe_unused = stage2_pmd_offset(kvm, pud, 0);
>> VM_BUG_ON(stage2_pud_huge(kvm, *pud));
>> stage2_pud_clear(kvm, pud);
>> - kvm_tlb_flush_vmid_ipa(mmu, addr);
>> + kvm_tlb_flush_vmid_ipa(mmu, addr, S2_NO_LEVEL_HINT);
>> stage2_pmd_free(kvm, pmd_table);
>> put_page(virt_to_page(pud));
>> }
>> @@ -186,7 +187,7 @@ static void clear_stage2_pmd_entry(struct kvm_s2_mmu *mmu, pmd_t *pmd, phys_addr
>> pte_t *pte_table = pte_offset_kernel(pmd, 0);
>> VM_BUG_ON(pmd_thp_or_huge(*pmd));
>> pmd_clear(pmd);
>> - kvm_tlb_flush_vmid_ipa(mmu, addr);
>> + kvm_tlb_flush_vmid_ipa(mmu, addr, S2_NO_LEVEL_HINT);
>> free_page((unsigned long)pte_table);
>> put_page(virt_to_page(pmd));
>> }
>
> Going by the names, is it possible to give a better level hint for these
> cases?

There is no leaf entry being invalidated here. After clearing the range, we found we'd
emptied (and invalidated) a whole page of mappings:
| if (stage2_pmd_table_empty(kvm, start_pmd))
| clear_stage2_pud_entry(mmu, pud, start_addr);

Now we want to remove the link to the empty page so we can free it. We are changing the
structure of the tables, not what gets mapped.

I think this is why we need the un-hinted behaviour, to invalidate "any level of the
translation table walk required to translate the specified IPA". Otherwise the hardware
can look for a leaf at the indicated level, find none, and do nothing.

This is sufficiently horrible, its possible I've got it completely wrong! (does it make
sense?)

Thanks,

James