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

[Marc Zyngier]

On Wed, 11 Aug 2021 10:37:36 +0100,
Anshuman Khandual <anshuman.khandual@xxxxxxx> wrote:
> 
> 
> 
> 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)

I'm sorry, but that's only a build system artefact. PAGE_SIZE/SHIFT is
what we use in the kernel at large, not IS_ENABLED(BLAH). It is short,
to the point, and it is guaranteed to be what it says on the tin.

If by some miracle you were going to enable multiple *simultaneous*
page sizes support in the arm64 kernel, I'd certainly look at things
differently. Thankfully, this isn't the case.

> 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));

3 instances are hardly a convincing argument.

maz@hot-poop:~/arm-platforms$ git grep -w PAGE_SIZE | grep  '== SZ'
arch/powerpc/include/asm/nohash/32/mmu-8xx.h:	if (PAGE_SIZE == SZ_16K)
fs/btrfs/disk-io.c:	if ((PAGE_SIZE == SZ_4K && sectorsize != PAGE_SIZE) ||
fs/btrfs/disk-io.c:	    (PAGE_SIZE == SZ_64K && (sectorsize != SZ_4K &&
fs/btrfs/disk-io.c:	if (PAGE_SIZE == SZ_64K && sectorsize == SZ_4K) {

Look, I win! :-)

> > 
> > 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.

It really doesn't. This is only a bike-shedding exercise, which
introduce pointless churn.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.