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

From: Andrew Scull <[email-masked]>
Date: Wed, 13 May 2020 10:06:48 +0100

On Tue, May 12, 2020 at 01:04:31PM +0100, James Morse wrote:
> 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?)

Ok. `addr` is an IPA, that IPA is now omitted from the map so doesn't
appear in any entry of the table, least of all a leaf entry. That makes
sense.

Is there a convention to distinguish IPA and PA similar to the
distinction for VA or does kvmarm just use phys_addr_t all round?

It seems like the TTL patches are failry self contained if it would be
easier to serparate them out from these larger series?

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