Re: [PATCH 1/5] KVM: arm64: Drop direct PAGE_[SHIFT|SIZE] usage as page size

[Anshuman Khandual]

On 8/11/21 1:41 PM, Marc Zyngier wrote:
> On Wed, 11 Aug 2021 06:34:46 +0100,
> Anshuman Khandual <anshuman.khandual@xxxxxxx> wrote:
>>
>>
>>
>> On 8/10/21 7:03 PM, Marc Zyngier wrote:
>>> On 2021-08-10 08:02, Anshuman Khandual wrote:
>>>> All instances here could just directly test against CONFIG_ARM64_XXK_PAGES
>>>> instead of evaluating via PAGE_SHIFT or PAGE_SIZE. With this change, there
>>>> will be no such usage left.
>>>>
>>>> Cc: Marc Zyngier <maz@xxxxxxxxxx>
>>>> Cc: James Morse <james.morse@xxxxxxx>
>>>> Cc: Alexandru Elisei <alexandru.elisei@xxxxxxx>
>>>> Cc: Suzuki K Poulose <suzuki.poulose@xxxxxxx>
>>>> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
>>>> Cc: Will Deacon <will@xxxxxxxxxx>
>>>> Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
>>>> Cc: kvmarm@xxxxxxxxxxxxxxxxxxxxx
>>>> Cc: linux-kernel@xxxxxxxxxxxxxxx
>>>> Signed-off-by: Anshuman Khandual <anshuman.khandual@xxxxxxx>
>>>> ---
>>>>  arch/arm64/kvm/hyp/pgtable.c | 6 +++---
>>>>  arch/arm64/mm/mmu.c          | 2 +-
>>>>  2 files changed, 4 insertions(+), 4 deletions(-)
>>>>
>>>> diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
>>>> index 05321f4165e3..a6112b6d6ef6 100644
>>>> --- a/arch/arm64/kvm/hyp/pgtable.c
>>>> +++ b/arch/arm64/kvm/hyp/pgtable.c
>>>> @@ -85,7 +85,7 @@ static bool kvm_level_supports_block_mapping(u32 level)
>>>>       * Reject invalid block mappings and don't bother with 4TB mappings for
>>>>       * 52-bit PAs.
>>>>       */
>>>> -    return !(level == 0 || (PAGE_SIZE != SZ_4K && level == 1));
>>>> +    return !(level == 0 || (!IS_ENABLED(CONFIG_ARM64_4K_PAGES) && level == 1));
>>>>  }
>>>>
>>>>  static bool kvm_block_mapping_supported(u64 addr, u64 end, u64 phys, u32 level)
>>>> @@ -155,7 +155,7 @@ static u64 kvm_pte_to_phys(kvm_pte_t pte)
>>>>  {
>>>>      u64 pa = pte & KVM_PTE_ADDR_MASK;
>>>>
>>>> -    if (PAGE_SHIFT == 16)
>>>> +    if (IS_ENABLED(CONFIG_ARM64_64K_PAGES))
>>>>          pa |= FIELD_GET(KVM_PTE_ADDR_51_48, pte) << 48;
>>>>
>>>>      return pa;
>>>> @@ -165,7 +165,7 @@ static kvm_pte_t kvm_phys_to_pte(u64 pa)
>>>>  {
>>>>      kvm_pte_t pte = pa & KVM_PTE_ADDR_MASK;
>>>>
>>>> -    if (PAGE_SHIFT == 16)
>>>> +    if (IS_ENABLED(CONFIG_ARM64_64K_PAGES))
>>>>          pte |= FIELD_PREP(KVM_PTE_ADDR_51_48, pa >> 48);
>>>>
>>>>      return pte;
>>>> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
>>>> index 9ff0de1b2b93..8fdfca179815 100644
>>>> --- a/arch/arm64/mm/mmu.c
>>>> +++ b/arch/arm64/mm/mmu.c
>>>> @@ -296,7 +296,7 @@ static void alloc_init_cont_pmd(pud_t *pudp,
>>>> unsigned long addr,
>>>>  static inline bool use_1G_block(unsigned long addr, unsigned long next,
>>>>              unsigned long phys)
>>>>  {
>>>> -    if (PAGE_SHIFT != 12)
>>>> +    if (!IS_ENABLED(CONFIG_ARM64_4K_PAGES))
>>>>          return false;
>>>>
>>>>      if (((addr | next | phys) & ~PUD_MASK) != 0)
>>>
>>> I personally find it a lot less readable.
>>>
>>> Also, there is no evaluation whatsoever. All the code guarded
>>> by a PAGE_SIZE/PAGE_SHIFT that doesn't match the configuration
>>> is dropped at compile time.
>>
>> The primary idea here is to unify around IS_ENABLED(CONFIG_ARM64_XXK_PAGES)
>> usage in arm64, rather than having multiple methods to test page size when
>> ever required.
> 
> I'm sorry, but I find the idiom extremely painful to parse. If you are

Okay, it was not explained very well. My bad.

> annoyed with the 'PAGE_SHIFT == 12/14/16', consider replacing it with
> 'PAGE_SIZE == SZ_4/16/64K' instead.

Sure, understood. But the problem here is not with PAGE_SHIFT/PAGE_SIZE
based tests but rather having multiple ways of doing the same thing in
arm64 tree. Please find further explanation below.

> 
> IS_ENABLED(CONFIG_ARM64_XXK_PAGES) also gives the wrong impression
> that *multiple* page sizes can be selected at any given time. That's
> obviously not the case, which actually makes PAGE_SIZE a much better
> choice.

PAGE_SHIFT and PAGE_SIZE are derived from CONFIG_ARM64_XXK_PAGES. Hence
why not just directly use the original user selected config option that
eventually decides PAGE_SHIFT and PAGE_SIZE.

config ARM64_PAGE_SHIFT
        int
        default 16 if ARM64_64K_PAGES
        default 14 if ARM64_16K_PAGES
        default 12

arch/arm64/include/asm/page-def.h:#define PAGE_SHIFT	CONFIG_ARM64_PAGE_SHIFT
arch/arm64/include/asm/page-def.h:#define PAGE_SIZE	(_AC(1, UL) << PAGE_SHIFT)

Also there are already similar IS_ENABLED() instances which do not
create much confusion. The point here being, to have just a single
method that checks compiled page size support, instead of three
different ways of doing the same thing.

- IS_ENABLED(CONFIG_ARM64_XXK_PAGES)
- if (PAGE_SHIFT == XX)
- if (PAGE_SIZE == XX)

$git grep IS_ENABLED arch/arm64/ | grep PAGES

arch/arm64/include/asm/vmalloc.h:	return IS_ENABLED(CONFIG_ARM64_4K_PAGES) &&
arch/arm64/mm/mmu.c:		BUG_ON(!IS_ENABLED(CONFIG_ARM64_16K_PAGES));
arch/arm64/mm/mmu.c:		BUG_ON(!IS_ENABLED(CONFIG_ARM64_16K_PAGES));

> 
> As things stand, I don't plan to take such a patch.

Sure, will drop it from the series if the above explanation and
the rationale for the patch still does not convince you.

> 
> Thanks,
> 
> 	M.
>