linux-kernel - Re: [PATCH 1/1] KVM: arm64: Use a separate function for hyp stage-1 walks

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Message-ID: <20221115132532.GA524@willie-the-truck>
Date:   Tue, 15 Nov 2022 13:25:34 +0000
From:   Will Deacon <will@...nel.org>
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>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        kvm@...r.kernel.org, kvmarm@...ts.linux.dev,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] KVM: arm64: Use a separate function for hyp stage-1
 walks

On Mon, Nov 14, 2022 at 08:11:27PM +0000, Oliver Upton wrote:
> A subsequent change to the page table walkers adds RCU protection for
> walking stage-2 page tables. KVM uses a global lock to serialize hyp
> stage-1 walks, meaning RCU protection is quite meaningless for
> protecting hyp stage-1 walkers.
> 
> Add a new helper, kvm_pgtable_hyp_walk(), for use when walking hyp
> stage-1 tables. Call directly into __kvm_pgtable_walk() as table
> concatenation is not a supported feature at stage-1.
> 
> No functional change intended.
> 
> Signed-off-by: Oliver Upton <oliver.upton@...ux.dev>
> ---
>  arch/arm64/include/asm/kvm_pgtable.h | 24 ++++++++++++++++++++++++
>  arch/arm64/kvm/hyp/nvhe/setup.c      |  2 +-
>  arch/arm64/kvm/hyp/pgtable.c         | 18 +++++++++++++++---
>  3 files changed, 40 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/kvm_pgtable.h b/arch/arm64/include/asm/kvm_pgtable.h
> index a874ce0ce7b5..43b2f1882e11 100644
> --- a/arch/arm64/include/asm/kvm_pgtable.h
> +++ b/arch/arm64/include/asm/kvm_pgtable.h
> @@ -596,6 +596,30 @@ int kvm_pgtable_stage2_flush(struct kvm_pgtable *pgt, u64 addr, u64 size);
>  int kvm_pgtable_walk(struct kvm_pgtable *pgt, u64 addr, u64 size,
>  		     struct kvm_pgtable_walker *walker);
>  
> +/**
> + * kvm_pgtable_hyp_walk() - Walk a hyp stage-1 page-table.
> + * @pgt:	Page-table structure initialized by kvm_pgtable_hyp_init().
> + * @addr:	Input address for the start of the walk.
> + * @size:	Size of the range to walk.
> + * @walker:	Walker callback description.
> + *
> + * The offset of @addr within a page is ignored and @size is rounded-up to
> + * the next page boundary.
> + *
> + * The walker will walk the page-table entries corresponding to the input
> + * address range specified, visiting entries according to the walker flags.
> + * Invalid entries are treated as leaf entries. Leaf entries are reloaded
> + * after invoking the walker callback, allowing the walker to descend into
> + * a newly installed table.
> + *
> + * Returning a negative error code from the walker callback function will
> + * terminate the walk immediately with the same error code.
> + *
> + * Return: 0 on success, negative error code on failure.
> + */
> +int kvm_pgtable_hyp_walk(struct kvm_pgtable *pgt, u64 addr, u64 size,
> +			 struct kvm_pgtable_walker *walker);

Hmm, this feels like slightly the wrong abstraction to me -- there's nothing
hyp-specific about the problem being solved, it's just that the only user
is for hyp walks.

Could we instead rework 'struct kvm_pgtable' slightly so that the existing
'flags' field is no-longer stage-2 specific and includes a KVM_PGTABLE_LOCKED
flag which could be set by kvm_pgtable_hyp_init()?

That way the top-level API remains unchanged and the existing callers will
continue to work.

Cheers,

Will