linux-kernel - Re: [PATCH v3 04/18] KVM: arm64: Move host page ownership tracking to the hyp vmemmap

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Message-ID: <86y10er2qb.wl-maz@kernel.org>
Date: Tue, 17 Dec 2024 11:03:08 +0000
From: Marc Zyngier <maz@...nel.org>
To: Quentin Perret <qperret@...gle.com>
Cc: 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>,
	Sebastian Ene <sebastianene@...gle.com>,
	linux-arm-kernel@...ts.infradead.org,
	kvmarm@...ts.linux.dev,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 04/18] KVM: arm64: Move host page ownership tracking to the hyp vmemmap

On Mon, 16 Dec 2024 17:57:49 +0000,
Quentin Perret <qperret@...gle.com> wrote:
> 
> We currently store part of the page-tracking state in PTE software bits
> for the host, guests and the hypervisor. This is sub-optimal when e.g.
> sharing pages as this forces to break block mappings purely to support
> this software tracking. This causes an unnecessarily fragmented stage-2
> page-table for the host in particular when it shares pages with Secure,
> which can lead to measurable regressions. Moreover, having this state
> stored in the page-table forces us to do multiple costly walks on the
> page transition path, hence causing overhead.
> 
> In order to work around these problems, move the host-side page-tracking
> logic from SW bits in its stage-2 PTEs to the hypervisor's vmemmap.
> 
> Signed-off-by: Quentin Perret <qperret@...gle.com>
> ---
>  arch/arm64/kvm/hyp/include/nvhe/memory.h |   6 +-
>  arch/arm64/kvm/hyp/nvhe/mem_protect.c    | 100 ++++++++++++++++-------
>  arch/arm64/kvm/hyp/nvhe/setup.c          |   7 +-
>  3 files changed, 77 insertions(+), 36 deletions(-)
> 
> diff --git a/arch/arm64/kvm/hyp/include/nvhe/memory.h b/arch/arm64/kvm/hyp/include/nvhe/memory.h
> index 45b8d1840aa4..8bd9a539f260 100644
> --- a/arch/arm64/kvm/hyp/include/nvhe/memory.h
> +++ b/arch/arm64/kvm/hyp/include/nvhe/memory.h
> @@ -8,7 +8,7 @@
>  #include <linux/types.h>
>  
>  /*
> - * SW bits 0-1 are reserved to track the memory ownership state of each page:
> + * Bits 0-1 are reserved to track the memory ownership state of each page:
>   *   00: The page is owned exclusively by the page-table owner.
>   *   01: The page is owned by the page-table owner, but is shared
>   *       with another entity.
> @@ -43,7 +43,9 @@ static inline enum pkvm_page_state pkvm_getstate(enum kvm_pgtable_prot prot)
>  struct hyp_page {
>  	u16 refcount;
>  	u8 order;
> -	u8 reserved;
> +
> +	/* Host (non-meta) state. Guarded by the host stage-2 lock. */
> +	enum pkvm_page_state host_state : 8;

An enum as a bitfield? Crazy! :)

You probably want an assert somewhere that ensures that hyp_page is a
32bit quantity, just to make sure (and avoid hard to track bugs).

Thanks,

	M.

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