linux-kernel - Re: [PATCH 1/5] KVM: arm64: Drop direct PAGE

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Message-ID: <87zgto9z9i.wl-maz@kernel.org>
Date:   Wed, 11 Aug 2021 09:11:37 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     Anshuman Khandual <anshuman.khandual@....com>
Cc:     linux-arm-kernel@...ts.infradead.org,
        James Morse <james.morse@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, kvmarm@...ts.cs.columbia.edu,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/5] KVM: arm64: Drop direct PAGE_[SHIFT|SIZE] usage as page size

On Wed, 11 Aug 2021 06:34:46 +0100,
Anshuman Khandual <anshuman.khandual@....com> 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@...nel.org>
> >> Cc: James Morse <james.morse@....com>
> >> Cc: Alexandru Elisei <alexandru.elisei@....com>
> >> Cc: Suzuki K Poulose <suzuki.poulose@....com>
> >> Cc: Catalin Marinas <catalin.marinas@....com>
> >> Cc: Will Deacon <will@...nel.org>
> >> Cc: linux-arm-kernel@...ts.infradead.org
> >> Cc: kvmarm@...ts.cs.columbia.edu
> >> Cc: linux-kernel@...r.kernel.org
> >> Signed-off-by: Anshuman Khandual <anshuman.khandual@....com>
> >> ---
> >>  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
annoyed with the 'PAGE_SHIFT == 12/14/16', consider replacing it with
'PAGE_SIZE == SZ_4/16/64K' instead.

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.

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

Thanks,

	M.

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