linux-kernel - Re: [PATCH v2 01/18] KVM: arm64: Change the layout of enum pkvm_page

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Message-ID: <CA+EHjTxfchr+ggO_z_zrrFTrFpS3oTJXt13t-854xWxYY=oSQQ@mail.gmail.com>
Date: Tue, 10 Dec 2024 12:59:44 +0000
From: Fuad Tabba <tabba@...gle.com>
To: Quentin Perret <qperret@...gle.com>
Cc: Marc Zyngier <maz@...nel.org>, Oliver Upton <oliver.upton@...ux.dev>, 
	Joey Gouly <joey.gouly@....com>, Suzuki K Poulose <suzuki.poulose@....com>, 
	Zenghui Yu <yuzenghui@...wei.com>, Catalin Marinas <catalin.marinas@....com>, 
	Will Deacon <will@...nel.org>, Vincent Donnefort <vdonnefort@...gle.com>, 
	Sebastian Ene <sebastianene@...gle.com>, linux-arm-kernel@...ts.infradead.org, 
	kvmarm@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 01/18] KVM: arm64: Change the layout of enum pkvm_page_state

Hi Quentin,

On Tue, 3 Dec 2024 at 10:37, Quentin Perret <qperret@...gle.com> wrote:
>
> The 'concrete' (a.k.a non-meta) page states are currently encoded using
> software bits in PTEs. For performance reasons, the abstract
> pkvm_page_state enum uses the same bits to encode these states as that
> makes conversions from and to PTEs easy.
>
> In order to prepare the ground for moving the 'concrete' state storage
> to the hyp vmemmap, re-arrange the enum to use bits 0 and 1 for this
> purpose.
>
> No functional changes intended.
>
> Signed-off-by: Quentin Perret <qperret@...gle.com>
> ---
>  arch/arm64/kvm/hyp/include/nvhe/mem_protect.h | 17 ++++++++++-------
>  1 file changed, 10 insertions(+), 7 deletions(-)
>
> diff --git a/arch/arm64/kvm/hyp/include/nvhe/mem_protect.h b/arch/arm64/kvm/hyp/include/nvhe/mem_protect.h
> index 0972faccc2af..ca3177481b78 100644
> --- a/arch/arm64/kvm/hyp/include/nvhe/mem_protect.h
> +++ b/arch/arm64/kvm/hyp/include/nvhe/mem_protect.h
> @@ -24,25 +24,28 @@
>   */
>  enum pkvm_page_state {
>         PKVM_PAGE_OWNED                 = 0ULL,
> -       PKVM_PAGE_SHARED_OWNED          = KVM_PGTABLE_PROT_SW0,
> -       PKVM_PAGE_SHARED_BORROWED       = KVM_PGTABLE_PROT_SW1,
> -       __PKVM_PAGE_RESERVED            = KVM_PGTABLE_PROT_SW0 |
> -                                         KVM_PGTABLE_PROT_SW1,
> +       PKVM_PAGE_SHARED_OWNED          = BIT(0),
> +       PKVM_PAGE_SHARED_BORROWED       = BIT(1),
> +       __PKVM_PAGE_RESERVED            = BIT(0) | BIT(1),
>
>         /* Meta-states which aren't encoded directly in the PTE's SW bits */
> -       PKVM_NOPAGE,
> +       PKVM_NOPAGE                     = BIT(2),
>  };
> +#define PKVM_PAGE_META_STATES_MASK     (~(BIT(0) | BIT(1)))
>
>  #define PKVM_PAGE_STATE_PROT_MASK      (KVM_PGTABLE_PROT_SW0 | KVM_PGTABLE_PROT_SW1)
>  static inline enum kvm_pgtable_prot pkvm_mkstate(enum kvm_pgtable_prot prot,
>                                                  enum pkvm_page_state state)
>  {
> -       return (prot & ~PKVM_PAGE_STATE_PROT_MASK) | state;
> +       BUG_ON(state & PKVM_PAGE_META_STATES_MASK);

This is a slight change in functionality, having a BUG_ON instead of
just masking out illegal states. Is it necessary?

Cheers,
/fuad


> +       prot &= ~PKVM_PAGE_STATE_PROT_MASK;
> +       prot |= FIELD_PREP(PKVM_PAGE_STATE_PROT_MASK, state);
> +       return prot;
>  }
>
>  static inline enum pkvm_page_state pkvm_getstate(enum kvm_pgtable_prot prot)
>  {
> -       return prot & PKVM_PAGE_STATE_PROT_MASK;
> +       return FIELD_GET(PKVM_PAGE_STATE_PROT_MASK, prot);
>  }
>
>  struct host_mmu {
> --
> 2.47.0.338.g60cca15819-goog
>