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

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Message-ID: <ZykFBhlzrUQtjEjy@google.com>
Date: Mon, 4 Nov 2024 17:31:50 +0000
From: Sebastian Ene <sebastianene@...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>, Fuad Tabba <tabba@...gle.com>,
	Vincent Donnefort <vdonnefort@...gle.com>,
	linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 01/18] KVM: arm64: Change the layout of enum
 pkvm_page_state

On Mon, Nov 04, 2024 at 01:31:47PM +0000, Quentin Perret wrote:

Hi Quentin,

> 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),
>  };

I guess we will still have to keep around the software bit annotation
for sharing MMIO regions from the host. This would not be tracked by the
vmemmap but it will still be in the s2 pagetable. As this is tagged with no
functional changes intended, are we safe because we are not supporting
MMIO sharing currently ?

> +#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);
> +	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);
>  }
>

Thanks,
Sebastian

>  struct host_mmu {
> --
> 2.47.0.163.g1226f6d8fa-goog
>