Re: [PATCH 12/17] kvm-arm: Add explicit hyp page table modifiers

[Marc Zyngier]

On 08/04/16 14:15, Christoffer Dall wrote:
> On Mon, Apr 04, 2016 at 05:26:12PM +0100, Suzuki K Poulose wrote:
>> We have common routines to modify hyp and stage2 page tables
>> based on the 'kvm' parameter. For a smoother transition to
>> using separate routines for each, duplicate the routines
>> and modify the copy to work on hyp.
>>
>> Marks the forked routines with _hyp_ and gets rid of the
>> kvm parameter which is no longer needed and is NULL for hyp.
>> Also, gets rid of calls to kvm_tlb_flush_by_vmid_ipa() calls
>> from the hyp versions. Uses explicit host page table accessors
>> instead of the kvm_* page table helpers.
>>
>> Suggested-by: Christoffer Dall <christoffer.dall@xxxxxxxxxx>
>> Cc: Marc Zyngier <marc.zyngier@xxxxxxx>
>> Signed-off-by: Suzuki K Poulose <suzuki.poulose@xxxxxxx>
>> ---
>>  arch/arm/kvm/mmu.c |  123 +++++++++++++++++++++++++++++++++++++++++++++++++---
>>  1 file changed, 118 insertions(+), 5 deletions(-)
>>
>> diff --git a/arch/arm/kvm/mmu.c b/arch/arm/kvm/mmu.c
>> index b46a337..2b491e5 100644
>> --- a/arch/arm/kvm/mmu.c
>> +++ b/arch/arm/kvm/mmu.c
>> @@ -388,6 +388,119 @@ static void stage2_flush_vm(struct kvm *kvm)
>>  	srcu_read_unlock(&kvm->srcu, idx);
>>  }
>>  
>> +static void clear_hyp_pgd_entry(pgd_t *pgd)
>> +{
>> +	pud_t *pud_table __maybe_unused = pud_offset(pgd, 0UL);
>> +	pgd_clear(pgd);
>> +	pud_free(NULL, pud_table);
>> +	put_page(virt_to_page(pgd));
>> +}
>> +
>> +static void clear_hyp_pud_entry(pud_t *pud)
>> +{
>> +	pmd_t *pmd_table __maybe_unused = pmd_offset(pud, 0);
>> +	VM_BUG_ON(pud_huge(*pud));
>> +	pud_clear(pud);
>> +	pmd_free(NULL, pmd_table);
>> +	put_page(virt_to_page(pud));
>> +}
>> +
>> +static void clear_hyp_pmd_entry(pmd_t *pmd)
>> +{
>> +	pte_t *pte_table = pte_offset_kernel(pmd, 0);
>> +	VM_BUG_ON(pmd_thp_or_huge(*pmd));
>> +	pmd_clear(pmd);
>> +	pte_free_kernel(NULL, pte_table);
>> +	put_page(virt_to_page(pmd));
>> +}
>> +
>> +static void unmap_hyp_ptes(pmd_t *pmd, phys_addr_t addr, phys_addr_t end)
>> +{
>> +	pte_t *pte, *start_pte;
>> +
>> +	start_pte = pte = pte_offset_kernel(pmd, addr);
>> +	do {
>> +		if (!pte_none(*pte)) {
>> +			pte_t old_pte = *pte;
>> +
>> +			kvm_set_pte(pte, __pte(0));
>> +
>> +			/* XXX: Do we need to invalidate the cache for device mappings ? */
> 
> no, we will not be swapping out any pages mapped in Hyp mode so you can
> get rid of both of the following two lines.
> 
>> +			if (!kvm_is_device_pfn(pte_pfn(old_pte)))
>> +				kvm_flush_dcache_pte(old_pte);
>> +
>> +			put_page(virt_to_page(pte));
>> +		}
>> +	} while (pte++, addr += PAGE_SIZE, addr != end);
>> +
>> +	if (hyp_pte_table_empty(start_pte))
>> +		clear_hyp_pmd_entry(pmd);
>> +}
>> +
>> +static void unmap_hyp_pmds(pud_t *pud, phys_addr_t addr, phys_addr_t end)
>> +{
>> +	phys_addr_t next;
>> +	pmd_t *pmd, *start_pmd;
>> +
>> +	start_pmd = pmd = pmd_offset(pud, addr);
>> +	do {
>> +		next = pmd_addr_end(addr, end);
>> +		if (!pmd_none(*pmd)) {
>> +			if (pmd_thp_or_huge(*pmd)) {
> 
> do we ever actually map anything with section mappings in the Hyp
> mappings?

No, this is purely a page mapping so far. On my system, the HYP text is
just over 4 pages big (4k pages), so the incentive is pretty low, unless
we can demonstrate some big gains due to the reduced TLB impact.

>> +				pmd_t old_pmd = *pmd;
>> +
>> +				pmd_clear(pmd);
>> +				kvm_flush_dcache_pmd(old_pmd);
>> +				put_page(virt_to_page(pmd));
>> +			} else {
>> +				unmap_hyp_ptes(pmd, addr, next);
>> +			}
>> +		}
>> +	} while (pmd++, addr = next, addr != end);
>> +
>> +	if (hyp_pmd_table_empty(start_pmd))
>> +		clear_hyp_pud_entry(pud);
>> +}
>> +
>> +static void unmap_hyp_puds(pgd_t *pgd, phys_addr_t addr, phys_addr_t end)
>> +{
>> +	phys_addr_t next;
>> +	pud_t *pud, *start_pud;
>> +
>> +	start_pud = pud = pud_offset(pgd, addr);
>> +	do {
>> +		next = pud_addr_end(addr, end);
>> +		if (!pud_none(*pud)) {
>> +			if (pud_huge(*pud)) {
> 
> do we ever actually map anything with huge pud
> mappings for the Hyp space?

Same thing. Looks like there is some potential simplification here.

> 
>> +				pud_t old_pud = *pud;
>> +
>> +				pud_clear(pud);
>> +				kvm_flush_dcache_pud(old_pud);
>> +				put_page(virt_to_page(pud));
>> +			} else {
>> +				unmap_hyp_pmds(pud, addr, next);
>> +			}
>> +		}
>> +	} while (pud++, addr = next, addr != end);
>> +
>> +	if (hyp_pud_table_empty(start_pud))
>> +		clear_hyp_pgd_entry(pgd);
>> +}
>> +
>> +static void unmap_hyp_range(pgd_t *pgdp, phys_addr_t start, u64 size)
>> +{
>> +	pgd_t *pgd;
>> +	phys_addr_t addr = start, end = start + size;
>> +	phys_addr_t next;
>> +
>> +	pgd = pgdp + pgd_index(addr);
>> +	do {
>> +		next = pgd_addr_end(addr, end);
>> +		if (!pgd_none(*pgd))
>> +			unmap_hyp_puds(pgd, addr, next);
>> +	} while (pgd++, addr = next, addr != end);
> 
> shouldn't we flush the EL2 (hyp) TLB here, strictly speaking?
> 
> Or do we rely on all mappings ever created/torn down here to always have
> the same VA/PA relationship?  Since we didn't flush the EL2 TLB in the
> existing code, that indeed does seem to be the case.

Actually, we never unmap anything from HYP. Once a structure (kvm, vcpu)
is mapped there, it stays forever, whatever happens to the VM (that's
because we'd otherwise have to refcount the number of objects in a page,
and I'm lazy...).

> That, in turn, raises the question why we don't simply map all pages
> that could be referenced by a kmalloc() in Hyp mode during the init
> phase and be done with all this hyp mapping/unmapping stuff?
> 
> In any case, that behavior doesn't have to change now, but if we don't
> add a TLB flush here, I'd like a comment to explain why that's not
> needed.

Hope you have your answer above... ;-)

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...