linux-kernel - Re: [PATCH 06/14] KVM: arm64: Return next table from map callbacks

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Date:   Wed, 7 Sep 2022 14:32:29 -0700
From:   David Matlack <dmatlack@...gle.com>
To:     Oliver Upton <oliver.upton@...ux.dev>
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>,
        Will Deacon <will@...nel.org>,
        Quentin Perret <qperret@...gle.com>,
        Ricardo Koller <ricarkol@...gle.com>,
        Reiji Watanabe <reijiw@...gle.com>,
        Ben Gardon <bgardon@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Gavin Shan <gshan@...hat.com>, Peter Xu <peterx@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 06/14] KVM: arm64: Return next table from map callbacks

On Tue, Aug 30, 2022 at 07:41:24PM +0000, Oliver Upton wrote:
> The map walkers install new page tables during their traversal. Return
> the newly-installed table PTE from the map callbacks to point the walker
> at the new table w/o rereading the ptep.
> 
> Signed-off-by: Oliver Upton <oliver.upton@...ux.dev>
> ---
>  arch/arm64/kvm/hyp/pgtable.c | 9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
> index 331f6e3b2c20..f911509e6512 100644
> --- a/arch/arm64/kvm/hyp/pgtable.c
> +++ b/arch/arm64/kvm/hyp/pgtable.c
> @@ -202,13 +202,12 @@ static inline int __kvm_pgtable_visit(struct kvm_pgtable_walk_data *data,
>  	if (!table && (flags & KVM_PGTABLE_WALK_LEAF)) {
>  		ret = kvm_pgtable_visitor_cb(data, addr, level, ptep, &pte,
>  					     KVM_PGTABLE_WALK_LEAF);
> -		pte = *ptep;
> -		table = kvm_pte_table(pte, level);
>  	}
>  
>  	if (ret)
>  		goto out;

Rather than passing a pointer to the local variable pte and requiring
all downstream code to update it (and deal with dereferencing to read
the old pte), wouldn't it be simpler to just re-read the PTE here? e.g.

        /*
         * Explicitly re-read the PTE since it may have been modified
         * during the TABLE_PRE or LEAF callback.
         */
        pte = kvm_pte_read(ptep);

This should also result in better behavior once parallelization is
introduced, because it will prevent the walker from traversing down and
doing a bunch of work on page tables that are in the process of being
freed by some other thread.

>  
> +	table = kvm_pte_table(pte, level);
>  	if (!table) {

nit: Technically there's no reason to set @table again. e.g. This could
just be:

        if (!kvm_pte_table(pte, level)) {

>  		data->addr = ALIGN_DOWN(data->addr, kvm_granule_size(level));
>  		data->addr += kvm_granule_size(level);
> @@ -427,6 +426,7 @@ static int hyp_map_walker(u64 addr, u64 end, u32 level, kvm_pte_t *ptep, kvm_pte
>  	new = kvm_init_table_pte(childp, mm_ops);
>  	mm_ops->get_page(ptep);
>  	smp_store_release(ptep, new);
> +	*old = new;
>  
>  	return 0;
>  }
> @@ -768,7 +768,7 @@ static int stage2_map_walker_try_leaf(u64 addr, u64 end, u32 level,
>  }
>  
>  static int stage2_map_walk_leaf(u64 addr, u64 end, u32 level, kvm_pte_t *ptep,
> -				struct stage2_map_data *data);
> +				kvm_pte_t *old, struct stage2_map_data *data);
>  
>  static int stage2_map_walk_table_pre(u64 addr, u64 end, u32 level,
>  				     kvm_pte_t *ptep, kvm_pte_t *old,
> @@ -791,7 +791,7 @@ static int stage2_map_walk_table_pre(u64 addr, u64 end, u32 level,
>  	 */
>  	kvm_call_hyp(__kvm_tlb_flush_vmid, data->mmu);
>  
> -	ret = stage2_map_walk_leaf(addr, end, level, ptep, data);
> +	ret = stage2_map_walk_leaf(addr, end, level, ptep, old, data);
>  
>  	mm_ops->put_page(ptep);
>  	mm_ops->free_removed_table(childp, level + 1, pgt);
> @@ -832,6 +832,7 @@ static int stage2_map_walk_leaf(u64 addr, u64 end, u32 level, kvm_pte_t *ptep,
>  	new = kvm_init_table_pte(childp, mm_ops);
>  	mm_ops->get_page(ptep);
>  	smp_store_release(ptep, new);
> +	*old = new;
>  
>  	return 0;
>  }
> -- 
> 2.37.2.672.g94769d06f0-goog
>