Re: [RFC 2/4] KVM: arm64: Support dirty page tracking for PUD hugepages

Punit Agrawal
2018-02-06T18:14:02Z

Christoffer Dall <christoffer.dall@linaro.org> writes:

> On Wed, Jan 10, 2018 at 07:07:27PM +0000, Punit Agrawal wrote:
>> In preparation for creating PUD hugepages at stage 2, add support for
>> write protecting PUD hugepages when they are encountered. Write
>> protecting guest tables is used to track dirty pages when migrating VMs.
>>
>> Also, provide trivial implementations of required kvm_s2pud_* helpers to
>> allow code to compile on arm32.
>>
>> Signed-off-by: Punit Agrawal <punit.agrawal@arm.com>
>> Cc: Christoffer Dall <christoffer.dall@linaro.org>
>> Cc: Marc Zyngier <marc.zyngier@arm.com>
>> ---
>> arch/arm/include/asm/kvm_mmu.h | 9 +++++++++
>> arch/arm64/include/asm/kvm_mmu.h | 10 ++++++++++
>> virt/kvm/arm/mmu.c | 9 ++++++---
>> 3 files changed, 25 insertions(+), 3 deletions(-)
>>
>> diff --git a/arch/arm/include/asm/kvm_mmu.h b/arch/arm/include/asm/kvm_mmu.h
>> index fa6f2174276b..3fbe919b9181 100644
>> --- a/arch/arm/include/asm/kvm_mmu.h
>> +++ b/arch/arm/include/asm/kvm_mmu.h
>> @@ -103,6 +103,15 @@ static inline bool kvm_s2pmd_readonly(pmd_t *pmd)
>> return (pmd_val(*pmd) & L_PMD_S2_RDWR) == L_PMD_S2_RDONLY;
>> }
>>
>> +static inline void kvm_set_s2pud_readonly(pud_t *pud)
>> +{
>> +}
>> +
>> +static inline bool kvm_s2pud_readonly(pud_t *pud)
>> +{
>> + return true;
>
> why true? Shouldn't this return the pgd's readonly value, strictly
> speaking, or if we rely on this never being called, have VM_BUG_ON() ?

It returns true as it prevents a call to kvm_set_s2pud_readonly() but
both of the above functions should never be called on ARM due to
stage2_pud_huge() returning 0.

I'll add a VM_BUG_ON(pud) to indicate that these functions should never
be called and...

>
> In any case, a comment explaining why we unconditionally return true
> would be nice.

... add a comment to explain what's going on.

>
>> +}
>> +
>> static inline bool kvm_page_empty(void *ptr)
>> {
>> struct page *ptr_page = virt_to_page(ptr);
>> diff --git a/arch/arm64/include/asm/kvm_mmu.h b/arch/arm64/include/asm/kvm_mmu.h
>> index 672c8684d5c2..dbfd18e08cfb 100644
>> --- a/arch/arm64/include/asm/kvm_mmu.h
>> +++ b/arch/arm64/include/asm/kvm_mmu.h
>> @@ -201,6 +201,16 @@ static inline bool kvm_s2pmd_readonly(pmd_t *pmd)
>> return kvm_s2pte_readonly((pte_t *)pmd);
>> }
>>
>> +static inline void kvm_set_s2pud_readonly(pud_t *pud)
>> +{
>> + kvm_set_s2pte_readonly((pte_t *)pud);
>> +}
>> +
>> +static inline bool kvm_s2pud_readonly(pud_t *pud)
>> +{
>> + return kvm_s2pte_readonly((pte_t *)pud);
>> +}
>> +
>> static inline bool kvm_page_empty(void *ptr)
>> {
>> struct page *ptr_page = virt_to_page(ptr);
>> diff --git a/virt/kvm/arm/mmu.c b/virt/kvm/arm/mmu.c
>> index 9dea96380339..02eefda5d71e 100644
>> --- a/virt/kvm/arm/mmu.c
>> +++ b/virt/kvm/arm/mmu.c
>> @@ -1155,9 +1155,12 @@ static void stage2_wp_puds(pgd_t *pgd, phys_addr_t addr, phys_addr_t end)
>> do {
>> next = stage2_pud_addr_end(addr, end);
>> if (!stage2_pud_none(*pud)) {
>> - /* TODO:PUD not supported, revisit later if supported */
>> - BUG_ON(stage2_pud_huge(*pud));
>> - stage2_wp_pmds(pud, addr, next);
>> + if (stage2_pud_huge(*pud)) {
>> + if (!kvm_s2pud_readonly(pud))
>> + kvm_set_s2pud_readonly(pud);
>> + } else {
>> + stage2_wp_pmds(pud, addr, next);
>> + }
>> }
>> } while (pud++, addr = next, addr != end);
>> }
>> --
>> 2.15.1
>>
>
> Otherwise:
>
> Reviewed-by: Christoffer Dall <christoffer.dall@linaro.org>

Thanks for taking a look.

Punit