[kvmarm] [PATCH v2 06/14] KVM: ARM: Memory virtualization setup

[Marc Zyngier]

On Sat, 6 Oct 2012 17:33:43 -0400, Christoffer Dall
<c.dall at virtualopensystems.com> wrote:
> On Thu, Oct 4, 2012 at 10:23 PM, Min-gyu Kim <mingyu84.kim at samsung.com>
> wrote:
>>
>>
>>> -----Original Message-----
>>> From: kvm-owner at vger.kernel.org [mailto:kvm-owner at vger.kernel.org] On
>>> Behalf Of Christoffer Dall
>>> Sent: Monday, October 01, 2012 6:11 PM
>>> To: kvm at vger.kernel.org; linux-arm-kernel at lists.infradead.org;
>>> kvmarm at lists.cs.columbia.edu
>>> Cc: Marc Zyngier
>>> Subject: [PATCH v2 06/14] KVM: ARM: Memory virtualization setup
>>>
>>> +static void stage2_set_pte(struct kvm *kvm, struct
kvm_mmu_memory_cache
>>> *cache,
>>> +                        phys_addr_t addr, const pte_t *new_pte) {
>>> +     pgd_t *pgd;
>>> +     pud_t *pud;
>>> +     pmd_t *pmd;
>>> +     pte_t *pte, old_pte;
>>> +
>>> +     /* Create 2nd stage page table mapping - Level 1 */
>>> +     pgd = kvm->arch.pgd + pgd_index(addr);
>>> +     pud = pud_offset(pgd, addr);
>>> +     if (pud_none(*pud)) {
>>> +             if (!cache)
>>> +                     return; /* ignore calls from kvm_set_spte_hva */
>>> +             pmd = mmu_memory_cache_alloc(cache);
>>> +             pud_populate(NULL, pud, pmd);
>>> +             pmd += pmd_index(addr);
>>> +             get_page(virt_to_page(pud));
>>> +     } else
>>> +             pmd = pmd_offset(pud, addr);
>>> +
>>> +     /* Create 2nd stage page table mapping - Level 2 */
>>> +     if (pmd_none(*pmd)) {
>>> +             if (!cache)
>>> +                     return; /* ignore calls from kvm_set_spte_hva */
>>> +             pte = mmu_memory_cache_alloc(cache);
>>> +             clean_pte_table(pte);
>>> +             pmd_populate_kernel(NULL, pmd, pte);
>>> +             pte += pte_index(addr);
>>> +             get_page(virt_to_page(pmd));
>>> +     } else
>>> +             pte = pte_offset_kernel(pmd, addr);
>>> +
>>> +     /* Create 2nd stage page table mapping - Level 3 */
>>> +     old_pte = *pte;
>>> +     set_pte_ext(pte, *new_pte, 0);
>>> +     if (pte_present(old_pte))
>>> +             __kvm_tlb_flush_vmid(kvm);
>>> +     else
>>> +             get_page(virt_to_page(pte));
>>> +}
>>
>>
>> I'm not sure about the 3-level page table, but isn't it necessary to
>> clean the page table for 2nd level?
>> There are two mmu_memory_cache_alloc calls. One has following
>> clean_pte_table
>> and the other doesn't have.
> 
> hmm, it probably is - I couldn't really find the common case where
> this is done in the kernel normally (except for some custom loop in
> ioremap and idmap), but I added this fix:
> 
> diff --git a/arch/arm/kvm/mmu.c b/arch/arm/kvm/mmu.c
> index 5394a52..f11ba27f 100644
> --- a/arch/arm/kvm/mmu.c
> +++ b/arch/arm/kvm/mmu.c
> @@ -430,6 +430,7 @@ static void stage2_set_pte(struct kvm *kvm, struct
> kvm_mmu_memory_cache *cache,
>  		if (!cache)
>  			return; /* ignore calls from kvm_set_spte_hva */
>  		pmd = mmu_memory_cache_alloc(cache);
> +		clean_dcache_area(pmd, PTRS_PER_PMD * sizeof(pmd_t));
>  		pud_populate(NULL, pud, pmd);
>  		pmd += pmd_index(addr);
>  		get_page(virt_to_page(pud));

There ought to be a test of ID_MMFR3[23:20] to find out whether or not it
is useful to clean the dcache. Not sure if that's a massive gain, but it is
certainly an optimisation to consider for the kernel as a whole.

        M.
-- 
Fast, cheap, reliable. Pick two.