linux-kernel - Re: [PATCH 7/7] KVM: arm64: Use less bits for hyp

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Message-ID: <87h7ih1ugi.wl-maz@kernel.org>
Date:   Tue, 01 Jun 2021 16:21:49 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     Quentin Perret <qperret@...gle.com>
Cc:     will@...nel.org, james.morse@....com, alexandru.elisei@....com,
        catalin.marinas@....com, suzuki.poulose@....com,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        kernel-team@...roid.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 7/7] KVM: arm64: Use less bits for hyp_page refcount

On Thu, 27 May 2021 13:51:34 +0100,
Quentin Perret <qperret@...gle.com> wrote:
> 
> The hyp_page refcount is currently encoded on 4 bytes even though we
> never need to count that many objects in a page. Make it 2 bytes to save
> some space in the vmemmap.
> 
> As overflows are more likely to happen as well, make sure to catch those
> with a BUG in the increment function.
> 
> Signed-off-by: Quentin Perret <qperret@...gle.com>
> ---
>  arch/arm64/kvm/hyp/include/nvhe/gfp.h    | 2 ++
>  arch/arm64/kvm/hyp/include/nvhe/memory.h | 2 +-
>  2 files changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kvm/hyp/include/nvhe/gfp.h b/arch/arm64/kvm/hyp/include/nvhe/gfp.h
> index d420e5c0845f..a82f73faf41e 100644
> --- a/arch/arm64/kvm/hyp/include/nvhe/gfp.h
> +++ b/arch/arm64/kvm/hyp/include/nvhe/gfp.h
> @@ -24,6 +24,8 @@ struct hyp_pool {
>  
>  static inline void hyp_page_ref_inc(struct hyp_page *p)
>  {
> +	if (p->refcount == USHRT_MAX)
> +		BUG();

nit: BUG_ON(p->refcount == USHRT_MAX);

>  	p->refcount++;
>  }
>  
> diff --git a/arch/arm64/kvm/hyp/include/nvhe/memory.h b/arch/arm64/kvm/hyp/include/nvhe/memory.h
> index 3fe34fa30ea4..592b7edb3edb 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>
>  
>  struct hyp_page {
> -	unsigned int refcount;
> +	unsigned short refcount;
>  	unsigned short order;
>  };
>  
> -- 
> 2.31.1.818.g46aad6cb9e-goog
> 
> 

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