linux-kernel - Re: [PATCH v3 03/15] KVM: arm64: Refcount hyp stage-1 pgtable pages

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Message-ID: <20211209102924.GC1833@willie-the-truck>
Date:   Thu, 9 Dec 2021 10:29:24 +0000
From:   Will Deacon <will@...nel.org>
To:     Quentin Perret <qperret@...gle.com>
Cc:     Marc Zyngier <maz@...nel.org>, James Morse <james.morse@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        linux-kernel@...r.kernel.org, kernel-team@...roid.com
Subject: Re: [PATCH v3 03/15] KVM: arm64: Refcount hyp stage-1 pgtable pages

On Wed, Dec 01, 2021 at 05:03:57PM +0000, Quentin Perret wrote:
> To prepare the ground for allowing hyp stage-1 mappings to be removed at
> run-time, update the KVM page-table code to maintain a correct refcount
> using the ->{get,put}_page() function callbacks.
> 
> Signed-off-by: Quentin Perret <qperret@...gle.com>
> ---
>  arch/arm64/kvm/hyp/pgtable.c | 17 ++++++++++++++---
>  1 file changed, 14 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
> index f8ceebe4982e..768a58835153 100644
> --- a/arch/arm64/kvm/hyp/pgtable.c
> +++ b/arch/arm64/kvm/hyp/pgtable.c
> @@ -408,8 +408,10 @@ static bool hyp_map_walker_try_leaf(u64 addr, u64 end, u32 level,
>  		return false;
>  
>  	new = kvm_init_valid_leaf_pte(phys, data->attr, level);
> -	if (hyp_pte_needs_update(old, new))
> +	if (hyp_pte_needs_update(old, new)) {
>  		smp_store_release(ptep, new);
> +		data->mm_ops->get_page(ptep);

In the case where we're just updating software bits for a valid pte, doesn't
this result in us taking a spurious reference to the page?

> @@ -482,8 +485,16 @@ static int hyp_free_walker(u64 addr, u64 end, u32 level, kvm_pte_t *ptep,
>  			   enum kvm_pgtable_walk_flags flag, void * const arg)
>  {
>  	struct kvm_pgtable_mm_ops *mm_ops = arg;
> +	kvm_pte_t pte = *ptep;
> +
> +	if (!kvm_pte_valid(pte))
> +		return 0;
> +
> +	mm_ops->put_page(ptep);
> +
> +	if (kvm_pte_table(pte, level))
> +		mm_ops->put_page(kvm_pte_follow(pte, mm_ops));
>  
> -	mm_ops->put_page((void *)kvm_pte_follow(*ptep, mm_ops));
>  	return 0;

This looks pretty similar to the stage-2 walker now, but given how small the
functions are, I'm not sure we'd really gain much by abstracting the "pte
counted" check.

Will